gitlab CI: add a check for the "allow collaboration" checkbox on the MR

See the docs:
https://docs.gitlab.com/ee/user/project/merge_requests/allow_collaboration.html

Without this enabled, we cannot rebase before merging. Having a CI stage that
fails hopefully makes the contributors look at it and edit the checkbox
accordingly without another back/forth from a maintainer.

"allow_failure: true" because there's no guarantee the MR has been filed yet
when we get here (either slow typing or it's WIP that doesn't have an MR yet
anyway).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
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