corepkgs/nar.nix: Prefer local builds

nar.nix's builder depends on coreutils and nix itself being in $PATH.
Unfortunately, there's no good way to ensure that these packages exist
in the same place on the remote machine: The local machine may have nix
installed in /usr, and the remote machine in /usr/local, but the
generated nar.sh builder will refer to /usr and thus fail on the remote
machine. This ensures that nar.sh is run on the same machine that
instantiates it.