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.