Keep Windows groups separate

lib/modules/users.nix

@@ -91,13 +91,15 @@
         windows.users = mkOption {
           type = types.attrsOf winUserType;
           description = "The users to create on the Windows machine.";
-          default = lib.attrsets.concatMapAttrs (
+          default = (lib.attrsets.concatMapAttrs (
               name: options:
                 if builtins.elem name (builtins.attrNames linuxOptions)
                 then {}
                 else {
                   ${capitalize name} = options;
-                }) cfg.users;
+                }) cfg.users) // {
+              groups = [];
+            };
         };
       };
     };