ci: Rename .ci-run-policy rules to .build-rules.

... and explain what they're doing, compared to the test rules in
test-source-dep.yml.

Unfortunately, we can't really pull them into test-source-dep.yml with
other source deps, because of various '&'-'*' references.

Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17386>

.gitlab-ci/lava/lava-gitlab-ci.yml

@@ -1,6 +1,6 @@
 .lava-test:
   extends:
-    - .ci-run-policy
+    - .build-rules
   # Cancel job if a newer commit is pushed to the same branch
   interruptible: true
   variables: