Follow-up from "Add gitlab-pages unique url lookup" - use `let_it_be` in spec/requests/api/internal/pages_spec.rb

The following discussion from !109891 (merged) should be addressed:

• @group_9970_bot2 started a discussion: (+1 comment)

          let_it_be(:project) { create(:project) }

  Project creations are very slow. Use let_it_be, build or build_stubbed if possible. See testing best practices for background information and alternative options.