ci: fix rustfmt job rules

Specifically, fe8bc3f23e made the "default" case `manual`
instead of `never`, which is causing all staging pipelines to have that
job regardless of any file `changes`, and always `manual` making it
block pipelines.

Let's make it automatic for upstream pipelines (ie. staging and
push-to-main), and only run on file `changes`.

Fixes: fe8bc3f23e ("ci: Only run rustfmt when necessary")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36260>

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

@@ -228,6 +228,9 @@
     - if: *is-merge-request
       when: never
     # non-pre-merge: run but only advisory
+    - if: $CI_PROJECT_NAMESPACE == "mesa"
+      changes: *rustfmt_files
+      allow_failure: true
     - when: manual
       allow_failure: true