Inherit artifacts through indirect inheritance.
Since fixing #1246 (closed):
If a user creates a new repository on debusine.debian.net, it will inherit from the developers workspace but not base (which developers inherits from).
Either we should make way for create_child_workspace to configure inheritance, or we should make indirect inheritance behave identically to direct inheritance for the artifact lookup.
This means implementing a CTE in visible_to_current_workspace https://salsa.debian.org/freexian-team/debusine/-/blob/0a9b06c0482446659128800f1fd187f54ba04653/debusine/db/models/artifacts.py#L96
It may be able to share code with this CTE: