tests: bump to 23.11

To have more "up-to-date" dependencies, should not change the CI status.

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Get the only thing that usually is reliable in CI
         uses: cachix/install-nix-action@v20
         with:
-          nix_path: nixpkgs=channel:nixos-23.05
+          nix_path: nixpkgs=channel:nixos-23.11
       - name: Update dependencies
         run: nix-shell -p elan --run "lake update"
       - name: Download mathlib4 cache