feat(arx): add experimental macos support via fakedir

[cassandracomar]

fixes #17

see fakedir for limitations and caveats.

I've tested the nix bundle interface and am able to produce a working executable arx. I need to find a machine that doesn't have the /nix/store paths already loaded to do a final validation. I tested using ruby and I'm able to run basic ruby scripts. I'm going to continue testing with a more complicated application that makes use of native extensions next.

basic flake for testing:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-24.05";
    flake-utils.url = "github:numtide/flake-utils";
    nix-bundle.url = "github:cassandracomar/nix-bundle";
    bundlers = {
      url = "github:nixos/bundlers";
      inputs.nix-bundle.follows = "nix-bundle";
    };
  };

  outputs = {
    nixpkgs,
    flake-utils,
    nix-bundle,
    bundlers,
    ...
  }:
    flake-utils.lib.eachDefaultSystem (
      system: let
        pkgs = import nixpkgs {
          inherit system;
        };
      in {
        defaultBundler = bundlers.defaultBundler.${system};
      }
    );
}

use this flake as your bundler:

nix bundle --bundler path:/path/to/flake nixpkgs#app

once this PR merges and we're able to update the nix-bundle reference in nixos/bundlers, nix bundle should work out of the box on macos, with the caveat that fakedir has to be able to intercept the syscalls in use by your application.

[cassandracomar]

I added a patch to fix a null pointer dereference when ruby calls getcwd() (it passes a null destination buffer to getcwd(), which I'm not sure is totally sensible). I'll upstream the patch when I figure out what to do about the fact that this patch technically leaks the buffer.

I've also made it possible to run multiple executables within the same target package (in the same /bin directory). it uses argv0 to decide which binary to execute so symlink the bottle to the other binaries you want to reach and use symlinkJoin to produce a composite package with all the tools you need in one place.

lastly, because arx changes PWD and because fakedir can't set up a chroot with paths bind mounted into the right places, I added a way to restore PWD context, so bottled tools work just like a natively installed tool -- there's now a TMPX_RESTORE_PWD env var set by the tmpx script before calling the startup script that you can use to reset the PWD context after saving the TMPX unpack dir.