nixpkgs-plugin-update: disable updateScript passthru

Causes error when running maintainer scripts for updating for a specific
maintainer.

"/nix/store/w35a8nwnl6vbrs7xf072iz4h0bzd9lrs-nix-update-1.14.0/lib/python3.13/site-packages/nix_update/update.py",
line 78, in fetch_new_version
    raise UpdateError(msg)
nix_update.errors.UpdateError: Could not find a url in the derivations
src attribute

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

pkgs/development/python-modules/nixpkgs-plugin-update/default.nix

@@ -32,6 +32,9 @@ buildPythonPackage {
     mypy
   '';
 
+  # NOTE: Causes "Could not find a url in the derivations src attribute" crash in maintainer scripts
+  passthru.updateScript = null;
+
   meta = {
     description = "Library for updating plugin collections in Nixpkgs";
     license = lib.licenses.mit;