meta: rerun a separate CI job

[vsemozhetbyt]

Sometimes CI has unrelated failures and I've seen other Collaborators rerun just separate failed jobs, not the full CI, to be sure. So there are 2 questions:

1. How can Collaborators do this? Should they just click on the job link and then press this button?
   
2. Should this case be detailed in the COLLABORATOR_GUIDE.md or onboarding.md?

[devsnek]

i never knew there was a way to rerun a specific job but i will probably do that in the future 😅

[Trott]

My opinion only, but my answer to both questions would be "yes." Use the Rebuild button (that's what I do, anyway) and this should probably be in one of those docs. (Probably doesn't matter much which one. We can always move it.)

[vsemozhetbyt]

Attempted: #20200

[joyeecheung]

You can also do this by filling out the parameters of that specific job (basically just need to fill out the refspec and the commit sha to rebase onto), except for the fanned ones.