Delete a weird nix version override (#3388)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: lf-

.github/workflows/nix.yml

@@ -57,8 +57,6 @@ jobs:
 
     - uses: cachix/install-nix-action@v18
       with:
-        install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
-        install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
         extra_nix_config: |
           experimental-features = nix-command flakes
         nix_path: nixpkgs=channel:nixos-unstable