ci: Always run 'build-assets-image' for master refs

Signed-off-by: Rémy Coutable <remy@rymai.me>

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

@@ -298,6 +298,7 @@
   rules:
     - <<: *if-not-canonical-namespace
       when: never
+    - <<: *if-master-refs
     - changes: *ci-build-images-patterns
     - changes: *code-qa-patterns