Enable merge-request pipelines
Without explicitly setting workflow rules, the pipelines in CI are triggered via a default "push" trigger.
Removing that trigger but adding in a merge request trigger instead will cause us to run "merge request pipelines" instead, which get other useful CI variables set, such as CI_MERGE_REQUEST_DIFF_BASE_SHA. https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html#add-jobs-to-merge-request-pipelines
This has to be merged to enable merge request pipelines in the project before we can test them in an MR that uses the new variables (!40 (merged)).
Progress on #40027 (closed)