gitlab-ci: Fix "triggered by Marge for a merge request" rule

The commit below changed the rule such that it accidentally also applied
to the non-MR pipelines created by Marge, resulting in Marge triggering
twice as many jobs as necessary.

Fixes: 549b4a3dd4ab "gitlab-ci: Automatically run pipelines for Marge
                     Bot pre-merge only"

Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5898>
1 file changed