matrix-appservice-discord: fix build by pinning nodejs_20

Build is failing with newer versions; From upstream's readme:

The bridge supports any version of Node.js between v18.X - v20.X.

pkgs/by-name/ma/matrix-appservice-discord/package.nix

···
   makeWrapper,
   removeReferencesTo,
   python3,
-  nodejs,
+  nodejs_20,
   matrix-sdk-crypto-nodejs,
 }:
 
 let
+  nodejs = nodejs_20; # only supports nodejs v18.X - v20.X
   pin = lib.importJSON ./pin.json;
   nodeSources = srcOnly nodejs;
 
···
 mkYarnPackage rec {
   pname = "matrix-appservice-discord";
   inherit (pin) version;
+  inherit nodejs;
 
   src = fetchFromGitHub {
     owner = "matrix-org";