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).