nixos/usbmuxd: Fix users.groups assignment

A mistake was introduced in https://github.com/NixOS/nixpkgs/pull/63103

Changed files
+1 -1
nixos
modules
services
hardware
+1 -1
nixos/modules/services/hardware/usbmuxd.nix
···
};
};
-
users.groups = optional (cfg.group == defaultUserGroup) {
+
users.groups = optionalAttrs (cfg.group == defaultUserGroup) {
${cfg.group} = { };
};