You can pass through the host FS using virtiofs to VMs
This basically allows the host to build packages which then appear in the VM(s)
Is there any solution already which implements nix* tools as proxy asking the parent machine to build the derivations ?
I’ve never heard of such a thing, but remote builders are a thing.
I’ve seen this done with containers, using a unix domain socket to talk to the host nix daemon, I think. For a VM you could use the ssh protocol to tally to the daemon I believe:
Alternatively possibly something with vsock sockets might be possible?