ci: Require manual execution of Rails spec jobs

With 9b1ef44c, the GitLab Rails suite of tests was introduced to the Gitaly pipeline and required to pass in order for merges to be accepted. Unfortunately, the Rails spec jobs are frequently flaky thus blocking merges.

Set the start-rails-specs CI job to only run if executed manually. Also remove the rules from rails-specs and rails-specs-cleanup so they execute automatically when start-rails-specs is triggered.