[vcpkg_download_distfile] fix download while in manifest root

[strega-nil]

When vcpkg detects a manifest root, it attempts to take a filesystem lock;
this is a bug in x-download.
We fix this bug by disabling manifests.

Fixes a bug in #13639

[strega-nil]

Example output before fix:

Detecting compiler hash for triplet arm64-osx...
The following packages will be built and installed:
    rapidxml[core]:arm64-osx -> 1.13-4
Starting package 1/1: rapidxml:arm64-osx
Building package rapidxml[core]:arm64-osx...
-- Using community triplet arm64-osx. This triplet configuration is not guaranteed to succeed.
-- [COMMUNITY] Loading triplet configuration from: /Users/nimazzuc/projects/vcpkg/triplets/community/arm64-osx.cmake
-- Downloading https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=cfhcable;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=pilotfiber;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=gigenet;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=versaweb;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=ayera;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=netactuate;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=phoenixnap;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=astuteinternet;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=freefr;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=netcologne;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=deac-riga;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=excellmedia;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=iweb;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=jaist;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=jztkft;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=managedway;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=nchc;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=netix;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=ufpr;https://sourceforge.net/projects/rapidxml/files/rapidxml/rapidxml%201.13/rapidxml-1.13.zip/download?use_mirror=tenet -> rapidxml-1.13.zip...
[DEBUG] Feature flag 'binarycaching' unset
[DEBUG] Feature flag 'manifests' unset
[DEBUG] Feature flag 'compilertracking' unset
[DEBUG] Feature flag 'registries' unset
[DEBUG] Feature flag 'versions' unset
[DEBUG] BuiltinRegistry initialized with: ""
[DEBUG] Using vcpkg-root: /Users/nimazzuc/projects/vcpkg
[DEBUG] Using manifest-root: /Users/nimazzuc/projects
[DEBUG] Using vcpkg_installed-root: /Users/nimazzuc/projects/vcpkg_installed
[DEBUG] Waiting to take filesystem lock on /Users/nimazzuc/projects/vcpkg/.vcpkg-root...
Failed to take the filesystem lock on /Users/nimazzuc/projects/vcpkg/.vcpkg-root:
    Resource busy
[DEBUG] /Users/nimazzuc/projects/vcpkg/buildtrees/_vcpkg/src/vcpkg-tool-2021-05-05-9f849c4c43e50d1b16186ae76681c27b0c1be9d9/src/vcpkg/vcpkgpaths.cpp(353)
[DEBUG] Exiting after 1557998 us (1521370 us)

CMake Error at scripts/cmake/vcpkg_download_distfile.cmake:221 (message):
      
      Failed to download file.
      If you use a proxy, please check your proxy setting. Possible causes are:
      
      1. You are actually using an HTTP proxy, but setting HTTPS_PROXY variable
         to `https://address:port`. This is not correct, because `https://` prefix
         claims the proxy is an HTTPS proxy, while your proxy (v2ray, shadowsocksr
         , etc..) is an HTTP proxy. Try setting `http://address:port` to both
         HTTP_PROXY and HTTPS_PROXY instead.
      
      2. You are using Fiddler. Currently a bug (https://github.com/microsoft/vcpkg/issues/17752)
         will set HTTPS_PROXY to `https://fiddler_address:port` which lead to problem 1 above.
         Workaround is open Windows 10 Settings App, and search for Proxy Configuration page,
         Change `http=address:port;https=address:port` to `address`, and fill the port number.
      
      3. You proxy's remote server is out of service.
      
      In future vcpkg releases, if you are using Windows, you no longer need to set
      HTTP(S)_PROXY environment variables. Vcpkg will simply apply Windows IE Proxy
      Settings set by your proxy software. See (https://github.com/microsoft/vcpkg-tool/pull/49)
      and (https://github.com/microsoft/vcpkg-tool/pull/77)
      
      Otherwise, please submit an issue at https://github.com/Microsoft/vcpkg/issues

Call Stack (most recent call first):
  scripts/cmake/vcpkg_from_sourceforge.cmake:158 (vcpkg_download_distfile)
  ports/rapidxml/portfile.cmake:2 (vcpkg_from_sourceforge)
  scripts/ports.cmake:139 (include)


Error: Building package rapidxml:arm64-osx failed with: BUILD_FAILED
Please ensure you're using the latest portfiles with `./vcpkg update`, then
submit an issue at https://github.com/Microsoft/vcpkg/issues including:
  Package: rapidxml:arm64-osx
  Vcpkg version: 2021-05-05-unknownhash

Additionally, attach any relevant sections from the log files above.