Update nix

Author: msoos

.github/workflows/nix.yml

@@ -13,6 +13,7 @@ jobs:
     - uses: actions/checkout@v4
     - uses: cachix/install-nix-action@v31
       with:
+        install_url: https://releases.nixos.org/nix/nix-2.26.3/install
         nix_path: nixpkgs=channel:nixos-unstable
     - uses: cachix/cachix-action@v14
       with: