Rework CI (backport #696)

[mergify]

---

This is an automatic backport of pull request #696 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 3379e73 has failed:

On branch mergify/bp/humble/pr-696
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 3379e73.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/ci.yml
	new file:   .github/workflows/pr.yml

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   .github/workflows/nightly.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally