This PR is a continuation of 238177 (Sorry for the trouble)

flutter-unwrapped will now not come with engine artifacts in its cache directory.
To specify a different cache directory, set FLUTTER_CACHE_DIR

Before this PR, changing which artifacts Flutter will have required rebuilding Flutter. This is because Flutter's unwrapped derivation required symlinking the engine artifacts to the cache directory($out/bin/cache).

So this:

pkgs.flutter.override {
  supportsAndroid = false;
  supportsLinuxDesktop = true;
}

And this:

pkgs.flutter.override {
  supportsAndroid = true;
  supportsLinuxDesktop = true;
}

Would build Flutter twice, even though it's not necessary.

To fix this, this PR moved the cache directory to an environment variable(when specified). This allows changing which artifacts
Flutter sees at runtime, so no need to rebuild.

Additionally, the wrapper has been adjusted to this change and will now provide the engine artifacts through FLUTTER_CACHE_DIR.

Note

The sh file $out/bin/internal/shared.sh runs when launching Flutter and calls $FLUTTER_ROOT/bin/cache/ instead of our environment variable FLUTTER_CACHE_DIR.
I decided not to patch it since the script doesn't require engine artifacts(which are the only thing not added by the unwrapped derivation), so it shouldn't fail, and patching it will just be harder to maintain.

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.