Move the cloud-native-image trigger job into the test stage

This is because the CNG pipeline now relies on the assets
compiled here in the gitlab:assets:compile job (It waits
up to 30 minutes for them)

Without this change, the cloud native job was holding up
the pipeline, preventing the assets compile job from being
run.

.gitlab-ci.yml

@@ -327,7 +327,7 @@ review-docs-cleanup:
 cloud-native-image:
   image: ruby:2.4-alpine
   before_script: []
-  stage: build
+  stage: test
   allow_failure: true
   variables:
     GIT_DEPTH: "1"