nixos/tests/hardened: fix eval

nixos/tests/hardened.nix

···
         };
       };
       boot.extraModulePackages =
-        optional (versionOlder config.boot.kernelPackages.kernel.version "5.6")
+        pkgs.lib.optional (pkgs.lib.versionOlder config.boot.kernelPackages.kernel.version "5.6")
           config.boot.kernelPackages.wireguard;
       boot.kernelModules = [ "wireguard" ];
     };