Running tests `on: push` prevents GitHub from showing the workflow approval
button, which prevents tests from being run on community-contributed (or even
just non-Nomad HashiCorp folks) PRs. Running `on: pull_request` automatically
picks up opened, reopened, and synchronize hooks (where "synchronize" means a
push to HEAD on the PR's branch, so that'll pick up rebases and updates).
But we also want to run tests on `main` and the various `release` backport
branches, so retain a `on: push` for those.
bbafb87e50