nix-build: --add-root also takes 1 parameter

scripts/nix-build.in

@@ -93,7 +93,7 @@ for (my $n = 0; $n < scalar @ARGV; $n++) {
         $n += 2;
     }
 
-    elsif ($arg eq "--max-jobs" || $arg eq "-j" || $arg eq "--max-silent-time" || $arg eq "--log-type" || $arg eq "--cores" || $arg eq "--timeout") {
+    elsif ($arg eq "--max-jobs" || $arg eq "-j" || $arg eq "--max-silent-time" || $arg eq "--log-type" || $arg eq "--cores" || $arg eq "--timeout" || $arg eq '--add-root') {
         $n++;
         die "$0: `$arg' requires an argument\n" unless $n < scalar @ARGV;
         push @buildArgs, ($arg, $ARGV[$n]);