Move cloud-native-image to the post-test stage

What does this MR do?

Moves the cloud-native=image job to the post-test stage from test

There was a race condition with the gitlab-assets-compile job which is also part of the test stage.

What are the relevant issue numbers?

Closes https://gitlab.com/charts/gitlab/issues/897

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.