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
15 jobs for topic/stable/ci-fix in 48 minutes and 27 seconds (queued for 4 seconds)