Fetch source from remote machine

[Greg Hogan]

The "build daemon can offload derivation builds to other machines
running Guix" [1], except source derivations look to always be fetched
from the host machine.

Would it be feasible to optionally allow source derivations to be
fetched on an offload machine for cases where the host machine does
not have full internet connectivity?

[1] https://guix.gnu.org/manual/devel/en/html_node/Daemon-Offload-Setup.html

Thanks,
Greg