buildNimPackage: add windows to platforms

since fixing the nixbuild patch to un-break cross compilation for
windows, it is now possible to use buildNimPackage to build windows binaries

pkgs/development/compilers/nim/default.nix

···
         meta = nim'.meta // {
           description = nim'.meta.description
             + " (${targetPlatformConfig} wrapper)";
-          platforms = with lib.platforms; unix ++ genode;
+          platforms = with lib.platforms; unix ++ genode ++ windows;
         };
       });
 in {