CI: Run a detached pipeline for merge requests

After abuses of fdo infrastructure were mitigated in
freedesktop/freedesktop#540, contributors cannot usually run pipelines
in their own forks of dbus.

Signed-off-by: Simon McVittie <smcv@collabora.com>

.gitlab-ci.yml

@@ -20,6 +20,11 @@
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_PIPELINE_SOURCE == 'push'
+
 include:
   - project: 'freedesktop/ci-templates'
     ref: e9a461877f8c7f9fed9fff5491067ec3c3472559