coqbot could add `suggested: rebase` when it might fix CI failures

[JasonGross]

When checks finish at the head of a branch in Coq, all open PR with a CI failure on ci-XXX which also fails on the CI of the base commit, but not on the new head could get a suggested: rebase label (probably not a comment, because that would flood people with multiple open PRs) (and the checks status for those CI jobs could be updated to indicate that rebasing might fix the CI run)