bug#51991: guix deploy with (with build-locally #f) needlessly syncs build artifacts

[Maxim Cournoyer]

Hello,

While configuring a machine setup for building the packages remotely, I
observed that all substitutes were first retrieved on the local host.

Since the intent is to deploy the remote machine and have it build the
package itself, this is a wasteful use resources.  The remote machine
should fetch the or substitutes or package sources itself.

Thanks,

Maxim