Convert the `working-directory` input in the release workflow from a
free-text string to a dropdown of known package paths.
## Changes
- Change `working-directory` from `type: string` to `type: choice` in
`_release.yml`, enumerating all 21 releasable packages under `libs/` and
`libs/partners/`
- Add `check-release-options` CI job in `check_diffs.yml` that runs a
pytest script to assert the dropdown options match directories
containing a `pyproject.toml`
