Support extra_repositories in sbuild workflow

Please add support for extra_repositories to the sbuild workflow. It should be relatively straight forward as the field can just be forwarded to child work requests.

We need the same for the debian_pipeline, but there it becomes less obvious of how it should be implemented. In principle, it could be prefixed with sbuild_ and forwarded in the same way, but then piuparts and other work requests likely also need to consume the extra repositories passed. If it turns out to be non-obvious to do this for the debian_pipeline please split the issue into two and do the sbuild workflow first.