ci: Fix job rules for `rails-specs` and `rails-specs-cleanup`

For the CI pipeline to run correctly the rail-specs job needs the start-rails-specs job to run and the rails-specs-cleanup job needs the rails-specs job to run. These jobs have mismatched rules though preventing them from executing in the same context which can lead to pipeline errors.

Make the CI rules for the three jobs all use rules_run_on_merge to ensure they run in the same context.