Revert "nixos: remove rsync from base install and add explicit path in nixos-install"

This reverts commit 582313ba.

Removing rsync is actually pointless because nixos-install depends on
it. So if it's part of the system closure, we may as well provide it
to users.

Probably with the next Nix release we can drop the use of rsync and
use "nix copy" instead.