gitlab-ci: Only trigger test-docs job automatically for MRs

Follow-up to
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5898 .

Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5918>
1 file changed