gitlab CI: move MR check to a later stage

This gives the developer enough time to file an MR after pushing a branch.
Having this run in the first stage means we get false positives because no MR
has been filed yet when the job is run.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
50 jobs for wip/ci-move-mr-check in 3 minutes and 24 seconds (queued for 6 seconds)