flake: add flake check which runs some feather tests

[LunNova]

This reuses the same setup hook for the dev shell LD_LIBRARY_PATH and C include path, so in theory the tests and the dev shell should break/work at the same time.

(env:feathergui) $ nix flake show                                                                                                                                                                      git+file:///home/lun/sync/dev/fundament/feathergui?ref=refs%2fheads%2fadd-flake-check&rev=1d777d73c6c7ba15c32868eb24912c5b063743b6
├───checks
│   ├───aarch64-linux
│   │   └───feather-tests: derivation 'feather-tests'
│   └───x86_64-linux
│       └───feather-tests: derivation 'feather-tests'
├───devShell
│   ├───aarch64-linux: development environment 'nix-shell'
│   └───x86_64-linux: development environment 'nix-shell'
└───packages
    ├───aarch64-linux
    │   ├───fgOpenGL: package 'backends'
    │   └───sail: package 'sail'
    └───x86_64-linux
        ├───fgOpenGL: package 'backends'
        └───sail: package 'sail'

(env:feathergui) $ nix flake check                                                                                                                                                                     warning: flake output attribute 'devShell' is deprecated; use 'devShells.<system>.default' instead
# no output, no errors

This also fixes nix flake check failing during eval due to trying to support systems scopes isn't available for, which looked like this on main:

(env:feathergui) $ nix flake check                                                                                                                                                                     warning: flake output attribute 'devShell' is deprecated; use 'devShells.<system>.default' instead
error: attribute 'aarch64-darwin' missing

       at /nix/store/pl2i4k1wcqgzia5g380j37p9xpnkrkqw-source/flake.nix:69:13:

           68|           buildInputs = [
           69|             scopes.packages.${system}.scopes
             |             ^
           70|             backends
(use '--show-trace' to show detailed location information)