Sync workflows main -> dev -> vX.Y-dev need app token

The `schema-test` workflow now only runs on pull_request because it needs to know the base branch (🙄) and no longer gets triggered for the sync PRs.

Fix seems to be to use an app token instead of the default GitHub token when creating the sync PRs, similar to what we do when creating spec publishing PRs from the Arazzo and Overlays repositories.