fcitx: show available engines in module description

nixos/modules/i18n/inputMethod/fcitx.nix

···
         type    = with types; listOf fcitxEngine;
         default = [];
         example = literalExample "with pkgs.fcitx-engines; [ mozc hangul ]";
-        description = ''
-          Enabled Fcitx engines.
-          Available engines can be found by running `nix-env "<nixpkgs>" . -qaP -A fcitx-engines`.
-        '';
+        description =
+          let
+            engines =
+              lib.concatStringsSep ", "
+              (map (name: "<literal>${name}</literal>")
+               (lib.attrNames pkgs.fcitx-engines));
+          in
+            "Enabled Fcitx engines. Available engines are: ${engines}.";
       };
     };