"fixup!" tests are ignored.

[chrysn]

Description

As part of #20286, a "fixup!" commit got merged by the merge queue.

Sorry!

Steps to reproduce the issue

Try merging a fixup commit, I guess -- but it's not like we have a place where we can dry-run this.

Expected results

The check-commits-success check (which did not pass because it depends on check-commits (pr_check)) should have been a critical check and thus prevented merging.

GitHub's settings for the master branch do show chekc-commits-success as required.

Actual results

The commit is now part of our master branch's history.

It's not the first; last time that happened was in 2023-09 (95d1e47).