Use treefmt to nixpkgs-fmt all .nix files

[flokli]

Checklist

• Add a new entry in an appropriate subdirectory of changelog.d
• Read and follow the PR guidelines

[jschaul]

Looks ok modulo the github actions block.

[jschaul]

This github action is unnecessary, as linters already run make lint-all here: https://github.com/zinfra/cailleach/blob/ab3a921c7a6cc903c0d1d578040e05a566c84bfa/wire-server-private/ci/pipelines/jobs/linters.dhall#L40-L60

please remove this github actions block. Also, we agreed to not introduce more github actions if not strictly necessary.

[flokli]

I'd like to keep these 20 lines of yaml.

• They're not a big maintenance burden, as it simply invokes treefmt after installing the dependencies.
• Very quick feedback (40 seconds)
• No requirement to log into concourse, so useful feedback for external contributors

It would be a different thing if we'd start to model complicated logic in GH actions, but this simply executes a script to give semi-instant feedback.

It looks like Concourse also does it, because it's invoked in the lint-all target from the Makefile, but it's way less accessible.

[jschaul]

it spams emails and re-does something that is already done, giving twice the amount of linting feedback for the same issue. To me, this is just noise and unnecessary. Maybe someone else has an opinion here also.

[flokli]

You can change if GH actions should send you emails on failure in your notification settings: