Fix data race with lookup paths

What does this MR do?

Since the API response for domains is cached, it's possible that multiple clients for the same host may cause a data race when sorting the lookup paths. To ensure this doesn't happen, we sort the lookup paths once before we cache it.

Relates to #646 (closed)

See investigation details in https://gitlab.com/gitlab-org/gitlab-pages/-/issues/690#note_1036454543.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines