Add a CI check to confirm that `nixpkgs-fmt` was run

[srid]

Either do it in a simple way or use the treefmt-nix flake-parts module (without bringing that flake input to users of haskell-flake).

[roberth]

The inputs concern would be an instance of

• https://github.com/hercules-ci/flake-parts/issues/119

[srid]

I think we can just use https://github.com/cachix/pre-commit-hooks.nix - which gives us a flake check for free.

EDIT: But of course, this would still require solving the dev-only dependencies part.