ci: fix rustfmt job rules (one more case)

f065c2f0ac actually always triggers the rule it adds because
that commit itself changes one of these files, so the case where none of
these files are changed was not actually tested.

The two options are to drop the `changes:` line from that commit, to
make every staging pipeline run that job, or to add the missing
"no changes" corresponding rule, which this commit does, which aligns
with the "run when files changed, don't run otherwise" behaviour of the
other rules above.

Fixes: f065c2f0ac ("ci: fix rustfmt job rules")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36306>

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

@@ -231,6 +231,8 @@
     - if: $CI_PROJECT_NAMESPACE == "mesa"
       changes: *rustfmt_files
       allow_failure: true
+    - if: $CI_PROJECT_NAMESPACE == "mesa"
+      when: never
     - when: manual
       allow_failure: true