gitlab-ci: Run lab tests automatically for MR pipelines

Add rule to run lab tests automatically when the pipeline is triggered
by a merge request event. This allows pickman-created MRs to have lab
tests run without needing to set SJG_LAB=1 manually.

Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

.gitlab-ci.yml

@@ -722,6 +722,8 @@ coreboot test.py:
   rules:
     - if: $SJG_LAB == $ROLE || $SJG_LAB == "1"
       when: always
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+      when: always
     - if: $SJG_LAB != "" && $SJG_LAB != "1" && $SJG_LAB != $ROLE
       when: never
     - if: $SJG_LAB == ""