CI: the version check in a workflow of its own #244

The version check runs as a job of build.yml, and build.yml listens to labeled/unlabeled so that the check sees the no-release label. Adding or removing any label therefore re-runs the whole build: unit tests, lint and both APKs.

Proposal, as in OpenIPC/camex#71: move the check to .github/workflows/version.yml, the only workflow that runs on label events; build.yml runs on the usual pull request events again. version.yml is CI, not build. CI only: no version bump.