gitlab-ci: Use single if for manual job rules entry

I thought multiple ifs would all need to match, but looks like only the
last one (or either one?) does.

This should prevent a manual pipeline from getting created after merging
changes which can't affect the pipeline.

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3474>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3474>

.gitlab-ci.yml

@@ -45,8 +45,7 @@ stages:
         *paths
       when: on_success
     # Allow triggering jobs manually on branches of forked projects
-    - if: '$CI_PROJECT_PATH != "mesa/mesa"'
-      if: '$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'
+    - if: '$CI_PROJECT_PATH != "mesa/mesa" && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME != $CI_COMMIT_REF_NAME'
       when: manual
     # Otherwise, most jobs won't run
     - when: never