ci: make sure pre-merge pipelines have the same jobs as merge pipelines

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26451>

.gitlab-ci.yml

@@ -219,10 +219,17 @@ include:
         - include/**/*
         - src/**/*
       when: on_success
+    # Same as above, but for pre-merge pipelines
+    - if: *is-pre-merge
+      changes:
+        *all_paths
+      when: manual
     # Just skip everything for MRs which don't actually change anything in the
     # build - the same rules as above, but without the file-change rules
     - if: *is-merge-attempt
       when: never
+    - if: *is-pre-merge
+      when: never
     # Build everything after someone bypassed the CI
     - if: *is-direct-push
       when: on_success