Use arrays in Ci::Pipeline#latest_builds_with_artifacts

What does this MR do?

This changes Ci::Pipeline#latest_builds_with_artifacts so it returns an Array instead of a relation, reducing the number of queries necessary to render the button for downloading CI artifacts. See the commit message for more details.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/36878