ci: complete buildbot migration

[trueNAHO]

This issue is a reminder and place for discussing the completion of the buildbot migration that started in https://github.com/nix-community/stylix/pull/1985.

Since buildbot only builds a subset of our GitHub CI, the GitHub CI cannot yet be removed.

For simplicity and buildbot's seeminlgy faster frontend and backend, all non-GitHub specific workflows should be migrated to buildbot. I suppose all workflows other than /.github/workflows/check.yml should not be migrated to buildbot. In that case, completing the buildbot migration requires feature parity with stylix-check by having it evaluate all tests.