$schema is always loaded from github even of available via file registry backend

The new schema library always tries to fetch the `$schema` file such as https://github.com/silverton-io/buz/blob/main/schemas/com.github/hook/repository/v1.0.json#L2 even if the schema is locally on disk provided by file registry backend.

removing the $schema file makes it work also within air gap environments.
