github-ci/nix: update to nixos-25.11

To pick up a newer version of Rust.

.github/workflows/nix.yml

@@ -12,7 +12,7 @@ jobs:
     - uses: actions/checkout@v7.0.1
     - uses: cachix/install-nix-action@v31
       with:
-        nix_path: nixpkgs=channel:nixos-24.11
+        nix_path: nixpkgs=channel:nixos-25.11
     - run: nix-shell --run ./scripts/bundle.sh
     - run: nix-shell --run ./autogen.sh
     - run: nix-shell --run "./configure CC=clang"