Trigger a 5.2 run every Monday morning

[jmid]

461 added a weekly trunk CI run.

To better understand

• whether some issues show up on 5.2 but not trunk, e.g, #446 and
• how the testsuite outcome works across time

this PR adds corresponding scheduled 5.2 CI runs every Monday morning.

[jmid]

CI summary:

• linux-s390x-5.1 failed with #444

Out of 36 workflows, 1 failed with a genuine issue

[jmid]

CI summary for merge to main:

• linux-s390x-5.1 timed out after 300m / 5h #421 - This was again on s390x-worker-01.marist.ci.dev.

Out of 37 workflows 1 failed with a borderline ocaml5/testsuite/CI issue