ci: move slow checkers last (#6668)
ci: move slow checkers last (#6668)

This PR reorders the checkers so that the slow ones are run last to give
the user more useful feedback quicker. It also makes the `shellcheck`
step run in parallel, which cuts its time in half. Moved a couple other
checks to more logical places in the order.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp