ci: Create new 'performance' stage

Move all jobs doing performance testing to a separate stage.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30784>

.gitlab-ci/test-source-dep.yml

@@ -91,6 +91,7 @@
 # Collabora triggers the manual job after merge to main.  These "never" filters
 # need to come before any paths with "manual".
 .performance-rules:
+  stage: performance
   rules:
     - !reference [.no_scheduled_pipelines-rules, rules]
     # Run only on pre-merge pipelines from Marge