Pull request checks into non-default branches use at most half of an account's CI/CD machines

Checks of pull requests whose base is a branch other than the repository's production branch now hold at most half of the machines the account can run at once, combined. The other half stays available for checks of the production branch and of pull requests into it, so those start immediately even when many pull requests into another branch are being checked. A check waiting on this limit shows the reason in its jobs: how many machines checks into other branches hold and the most they may hold.