qt5.mkDerivation: default enableParallelBuilding = true

pkgs/development/libraries/qt-5/mkDerivation.nix

···
           (if debug then "-DCMAKE_BUILD_TYPE=Debug"
                     else "-DCMAKE_BUILD_TYPE=Release");
 
-    enableParallelBuilding = args.enableParallelBuilding or false;
+    enableParallelBuilding = args.enableParallelBuilding or true;
 
   };
 in