Configurable virtual fs mounts based on environment variables

[phantamanta44]

This PR is a first step towards addressing #651. It allows users (or package maintainers) to customize the Etterna virtual filesystem mounts using environment variables so that the game installation itself can remain immutable while any variable data is stored elsewhere. This is especially necessary for a NixOS release, as the Nix store is read-only.

More specifically, this PR does the following:

• Causes Etterna to check the env var ETTERNA_ROOT_DIR on startup. If present, its value is used for the path of the main mount; otherwise, the current behaviour is used, namely mounting the installation directory.
• Causes Etterna to check the env var ETTERNA_ADDITIONAL_ROOT_DIRS on startup. If present, it's parsed as a list of paths that are treated identically to the paths listed in the "AdditionalFolders" setting. The list delimiter is the OS-dependent path separator, namely ; on Windows and : everywhere else.
• Causes Etterna to check the env var ETTERNA_ADDITIONAL_SONG_DIRS on startup. If present, it's parsed as a list of paths that are treated identically to the paths listed in the "AdditionalSongFolders" setting. As above, the separator is OS-dependent.
• Adds an ASSET_DIR CMake variable that can be configured to define where game assets should be installed. If absent, defaults to the current behaviour, namely to INSTALL_DIR. The purpose of this change is to allow assets to be separated from the rest of the game data so that they're easier to subsequently copy elsewhere. Could be useful for distribution on some Linux flavours.
• Makes INSTALL_DIR similarly configurable.
• Disables the CMake installation commands for crashpad binaries if Etterna is built without crashpad.

This last change comes up because the crashpad build task attempts to run built-in build tools (e.g. extern/crashpad/buildtools/linux64/gn) that fail to run on NixOS because of linking issues. Potentially, a more drastic refactoring of the build tools could fall back to the system installs of the build tools, but that wasn't a priority for me in this PR.

If these changes make it into release, the plan is to then PR nixpkgs with an expression for Etterna. The idea is to wrap the Etterna binary so that the game's root mount is bound to ~/.local/share/etterna in the user's home directory by environment variables, and then to add the game's immutable installation/asset directory as an additional mount.

[poco0317]

the changes look fine. does it work? i wont test it

[phantamanta44]

Tested and working on my amd64 nixos install, but that's the only system I have

[fgaz]

@phantamanta44 I was looking into packaging etterna for NixOS and found this. How's your package going?

[phantamanta44]

I have a nixpkgs expression ready to be PR'd, but I'm waiting on an official Etterna release that includes this patch

[fgaz]

When that happens feel free to ping me for a review

[poco0317]

although the repo activity is no indicator, it is likely that we will see a release in roughly a month. if you want to pr something to here open at any time. if you mean pr to nix or whatever just keep in mind that whatever you do for the love of god do not point a publicly available repackaging or distribution to anything other than master tags