gitlab-ci: Run merge request pipelines automatically only for Marge Bot

MR pipelines not triggered by Marge Bot can still be triggered manually.

Motivation: The main & forked Mesa project CI pipelines combined are currently generating over 1 TB of egress traffic per week. ~80% of this is from pre-merge pipelines. Assuming this corresponds to 4 pre-merge and one post-merge pipeline per MR on average, this change could potentially eliminate up to ~60% of the overall traffic (by preventing 3 of the 4 pre-merge pipelines from running automatically).

(Of course, this could be subverted if all jobs of the other pipelines were triggered manually anyway... In most cases, manually triggering just a few jobs should suffice)

Suggested-by: Marek Olšák maraeo@gmail.com