speed up Nix CI job by cache + flake check

[KiaraGrouwstra]

try and speed up Nix check in CI as discussed in #180

[KiaraGrouwstra]

ok i'm not sure this helped shave off time :(

[ErrorNoInternet]

An extremely hacky way is to possibly run each build phase manually, using nix develop, e.g. buildPhase.

And instead of using a Nix binary cache, you could use actions/cache to cache the target/ folder that buildPhase generates when calling cargo build.

Or if you guarantee that the devShell and package have the same inputs, you could just nix develop -c cargo b and use actions/cache to cache the target/ folder.

[KiaraGrouwstra]

i guess i'll close cuz https://github.com/sodiboo/niri-flake kinda addressed this