Lint and check formatting in CI

[iFreilicht]

Thanks to #716, we can very easily verify whether a PR has been properly formatted. But we're not doing it, yet. We would have to checkout the branch and run nix fmt -- --check manually.

Add a check to our CI to do this.

[Mic92]

In nixos-facter-modules we were able to include flakes for devshell/checks only:
https://github.com/numtide/nixos-facter-modules/blob/main/flake.nix

Given that all those flake follows are a bit annoying for end-users it might be worth the effort in project also.