CI/CD: checks a release waits on get machines first

When an account's checks are short of machines, a pull request from the repository's own branches into the production branch gets them first, then a push to the production branch, each oldest first, then every other check. While one of these waits, the account's lower checks hand it machines as their running tasks end. A pull request from a fork is ordered with every other check. To skip pull requests into a branch whose commits are tested another way, such as a staging branch, list the branches to check in the production branch's config: pull_requests: { branches: [main] }.