Run startup css as-if-foss if startup css has changed

- We have to update the `default-frontend-jobs...` rule
  because this is what the fixtures job uses. This will
  create some redundant jest as-if-foss runs, but it sounds
  like we're okay with this tradeoff.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63234#note_593916923

.gitlab/ci/rules.gitlab-ci.yml

@@ -150,6 +150,9 @@
   - "vendor/assets/**/*"
   - "{,ee/,jh/}{app/assets,app/helpers,app/presenters,app/views,locale,public,symbol}/**/*"
 
+.startup-css-patterns: &startup-css-patterns
+  - "{,ee/,jh/}app/assets/stylesheets/startup/**/*"
+
 .backend-patterns: &backend-patterns
   - "Gemfile{,.lock}"
   - "Rakefile"
@@ -439,6 +442,8 @@
       changes: *code-backstage-patterns
     - <<: *if-merge-request-title-as-if-foss
     - <<: *if-merge-request-title-run-all-rspec
+    - <<: *if-merge-request
+      changes: *startup-css-patterns
     - <<: *if-merge-request
       changes: *ci-patterns