ci: tag sanity, rustfmt and clang-format job as a "placeholder" job

There is close to zero work needed to execute this job.

Should speed up the initial process of entering into pipeline tree
and also provide an opportunity for `aarch64` runners to engage sooner,
even when x86_64 machines are loaded.

Acked-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26125>

.gitlab-ci.yml

@@ -265,6 +265,8 @@ sanity:
     when: on_failure
     reports:
       junit: check-*.xml
+  tags:
+    - placeholder-job
 
 
 # Jobs that need to pass before spending hardware resources on further testing

.gitlab-ci/test/gitlab-ci.yml

@@ -33,6 +33,8 @@
   timeout: 10m
   script:
     - git diff --color=always --exit-code  # Fails if there are diffs
+  tags:
+    - placeholder-job
 
 rustfmt:
   extends: