nixos-rebuild: use remote $PATH in buildHostCmd

this fixes the issue when using

    nixos-rebuild switch --target-host <target> --use-remote-sudo

when the local machine does not have anything in `$PATH` that would
resolve to `sudo` on the remote machine.

the single quotes prevent expansion of `$PATH` on the local machine,
such that the remote machine's value of that variable is used.

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -145,7 +145,7 @@ buildHostCmd() {
     if [ -z "$buildHost" ]; then
         "$@"
     elif [ -n "$remoteNix" ]; then
-        ssh $SSHOPTS "$buildHost" env PATH="$remoteNix:$PATH" "${maybeSudo[@]}" "$@"
+        ssh $SSHOPTS "$buildHost" env PATH="$remoteNix":'$PATH' "${maybeSudo[@]}" "$@"
     else
         ssh $SSHOPTS "$buildHost" "${maybeSudo[@]}" "$@"
     fi