CI/CD: built-in history checks finish immediately on production pushes

On a push to the production branch, an undoes or hot-spots step now passes as soon as the plan is made, because the pull request was already checked. It no longer waits for a sandbox. On a pull request, these steps still run on the check's light sandbox. While that sandbox, or the one that runs gates, waits for capacity, the console shows "Waiting for room" on each of its jobs, with the elapsed minutes updated every 5 minutes. Previously these jobs showed no update while they waited, and a check whose jobs waited more than 40 minutes failed with "sent nothing for 40 minutes".