Fixed wrong username variable insersion in configuration.nix

modules/configuration.nix

@@ -228,7 +228,7 @@
   # services.xserver.libinput.enable = true;
 
   # Define a user account. Don't forget to set a password with ‘passwd’.
-  users.users."{$username}" = {
+  users.users.${username} = {
     isNormalUser = true;
     description = "Cookiez";
     extraGroups = [ "networkmanager" "wheel" "docker" ];