Note that changing the origin repository in the on-node deployment checkout
is also required.

See https://whetstone.private.storage/privatestorage/privatestorageops/-/merge_requests/197#note_19071

This also update ZKAPAuthorizer to a python3 using version.

Thanks to @tomprince for the suggestion.

Thanks @tomprince!

The nixpkgs fork already contains all of the changes our overlays were
responsible for - but as direct nixpkgs changes instead.