python3Packages.materialx: disable auto updates

See e.g. bb4281dcaa and
27810f1018 where we had to revert, and
https://github.com/NixOS/nixpkgs/pull/394139 where every dependent
package is failing.

pkgs/development/python-modules/materialx/default.nix

@@ -18,6 +18,9 @@ buildPythonPackage rec {
   pname = "materialx";
   version = "1.38.10";
 
+  # nixpkgs-update: no auto update
+  # Updates are disabled due to API breakage in 1.39+ that breaks almost all
+  # consumers.
   src = fetchFromGitHub {
     owner = "AcademySoftwareFoundation";
     repo = "MaterialX";