Check Git user and fail early

[kettanaito]

If the Git user is not set, committing will fail. But since commit happens after the artifact has been published, it can fail the workflow and still create the release.

Strategy

1. Check for the Git user/email early. If not set, fail early. This can be checked alongside the GITHUB_* environment variable.
2. Consider exposing an API to create a release commit without making a release. Just a quality of life improvement to recover from this scenarios.