ci: do not trigger phabricator for merge-request The fast the phabricator steps has a `rules` entry makes it selected for the special `merge_requests` pipelines. The other ones behave as default and are not selected tot the mrege_request pipelines. This result in a second pipeline to be created, with only the phabricator pipeline in it. Which usually succeed fast (since there is nothing to do). This is harmful as this create a false sense of "the series is passing" and Gitlab will use this simplistic pipeline for validation. By explicitly preventing the pipeline to be created in the merge-request case, we prevent this situation to happens Note that the job will be dropped (alonside phabricator) in the next two weeks anyway.