Upgrade to nixpkgs 25.11

nixpkgs 25.11 was released on 2025-11-30: https://nixos.org/blog/announcements/2025/nixos-2511/
We should upgrade our nix setup.

 - I'll first update in mlkem-native and then port it to mldsa-native, see https://github.com/pq-code-package/mlkem-native/issues/1355