minor fix in CI job execution rules

.gitlab-ci.yml

@@ -191,7 +191,7 @@ build-spack-env-on-runner:
     when: always
   timeout: 2 days
   rules:
-    - if: $CI_PIPELINE_SOURCE != "schedule"
+    - if: $CI_PIPELINE_SOURCE != "schedule" && $CI_PIPELINE_SOURCE != "merge_request_event"
 
 sync-gitlab-spack-instance:
   stage: build