Easy GitHub Actions integration

[infinisil]

There should be some way for people to easily start enforcing official nixfmt formatting on their GitHub repos. A GitHub Action workflow would be great for that.

See NixOS/nixpkgs#299578 (comment) for more context

[gabyx]

Although Github actions are a good first solution, it would be nice to just have a nix run nixfmt#format <git-diff-args> or something along these lines, so that other people not using Github Actions also can use it directly in a Nixfied container.
If I would do this, I would directly head for a nix run ??? main...pr-branch thing and then add the Github Action which could only be (not sure) wrapping this nix call. So everybody with a CI, or a local workflow can just do that and it works.

The --check option would just do the thing in a temp. folder and report if diffs happened or not.

[infinisil]

Duplicate of #76

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/formatting-team-meeting-2024-04-02/42658/1