Add dedicated runner tags to assets job

Ordinarily it would get the `gitlab-org` tag from
`dedicated-no-docs-pull-cache-job`, but adding the `docker` tag
overwrites rather than inherits, so this job has been running on the
underpowered hardware of the shared runners rather than our dedicated
runners.

We're also adding a `high-cpu` tag, which doesn't currently do anything
but will be useful for targeting a runner specifically for this job.

.gitlab-ci.yml

@@ -620,7 +620,8 @@ gitlab:assets:compile:
       - webpack-report/
       - public/assets/
   tags:
-    - docker
+    - gitlab-org-delivery
+    - high-cpu
 
 karma:
   <<: *dedicated-no-docs-pull-cache-job