Have CI enforce Ruff linting rules for files modified by MRs

closes #20049 (closed) closes #19306 (closed)

the output of MR in this specific case is quite irrelevant, because only one python file have been changed. However, you can manually test this on your own, running, for example: ./bin/test-utils/ruff origin/merge-requests/1306 387cdb0769

(this would check how it would have behaved for !1306 (merged))

one file has been changed for the exact purpose of demonstrating it:

• 9547bdc8 shows that the job fails and it provides useful output
• the next commit fixes the problems, and the CI is happy