Fix Check Nix flake CI by updating stale nixpkgs pin.

The flake-checker rejects nixpkgs inputs older than 30 days; also disable FlakeHub in the cache action since this repo does not use it.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Scott at Work

.github/workflows/check-flake.yml

@@ -14,6 +14,8 @@ jobs:
       - uses: actions/checkout@v6
       - uses: DeterminateSystems/nix-installer-action@v22
       - uses: DeterminateSystems/magic-nix-cache-action@v13
+        with:
+          use-flakehub: false
       - name: Check Nix flake inputs
         uses: DeterminateSystems/flake-checker-action@v5
         with: