ci: added treefmt workflow to gha and removed from buildbot

[c4patino]

Seemed like low hanging fruit in #3561

• Migrates the treefmt foramtting check from buildbot to a dedicated Github Actions workflow.
• Removes the buildbot treefmt check from flake/dev/fmt.nix to avoid duplicate runs with the new formatter.

[MattSturgeon]

Thanks for working on this!

[c4patino]

Hi @MattSturgeon is there anything else that you need on this?

[MattSturgeon]

This is complicated a little by merge queues and "required status checks", however I'm fine merging this initially without adding it as a "required" check.

#3628 can then be responsible for implementing the required checks for PRs and merge queues.

A few minor things to address or discuss below, then I think we can get this merged.

Thanks again!

[MattSturgeon]

This name shows up in the status check list as Workflow name / Job name; in this case Formatting / Check formatting with treefmt.

Bike-shedding the names doesn't block this PR, because we can change them at any time in other PRs. But my personal preference would be to be less verbose and avoid redundancy between workflow & job names.

How about Formatting / treefmt or Lint / formatting or Check / Formatting?

[c4patino]

switching to lint / treefmt. forgot that those show up as really long. i've had problems with that before when using reusable workflows.

[MattSturgeon]

FYI you renamed the job id to lint, not the workflow name. The workflow name: Formatting is at the top of the file 🙃 (and should probably be somewhat in-sync with the actual filename, currently treefmt.yml, too).

Also, if the job name and id are the same, there's no need to set its name explicitly.

[c4patino]

thats embarassing. fixed. it should just be lint / treefmt now.