nixos/postfix: add missing setgid wrapper

This is basically an alias for a special case of postqueue, which
already has a setgid wrapper.  Would be silly to allow postqueue -p
but not mailq.

nixos/modules/services/mail/postfix.nix

@@ -631,6 +631,14 @@ in
         setgid = true;
       };
 
+      security.wrappers.mailq = {
+        program = "mailq";
+        source = "${pkgs.postfix}/bin/mailq";
+        group = setgidGroup;
+        setuid = false;
+        setgid = true;
+      };
+
       security.wrappers.postqueue = {
         program = "postqueue";
         source = "${pkgs.postfix}/bin/postqueue";