nixos-rebuild: add explicit option to enable (remote) sudo

Add --use-remote-sudo option. When set, remote commands will be prefixed
with 'sudo'. This allows using sudo remotely _without_ having to use
sudo locally (when using --build-host/--taget-host).

nixos/modules/installer/tools/nixos-rebuild.sh

@@ -90,6 +90,11 @@ while [ "$#" -gt 0 ]; do
         targetHost="$1"
         shift 1
         ;;
+      --use-remote-sudo)
+        # note the trailing space
+        maybeSudo="sudo "
+        shift 1
+        ;;
       *)
         echo "$0: unknown option \`$i'"
         exit 1
@@ -97,10 +102,6 @@ while [ "$#" -gt 0 ]; do
     esac
 done
 
-if [ -n "$SUDO_USER" ]; then
-    maybeSudo="sudo "
-fi
-
 if [ -z "$buildHost" -a -n "$targetHost" ]; then
     buildHost="$targetHost"
 fi