ci: Don’t run pipeline after merging a MR

MRs are already tested in CI before merge, so it’s redundant and a waste of resources to test them again after merge.

In the rare case where something breaks post-merge (perhaps because several MRs have been tested individually and merged, but interact with each other badly), that’ll be caught in the weekly scheduled CI run.

YAML inspiration from https://stackoverflow.com/questions/63893431/gitlab-run-a-pipeline-job-when-a-merge-request-is-merged

Signed-off-by: Philip Withnall pwithnall@endlessos.org