Bump Nix version for CD

Following the example from the install-nix-action repo:
cachix/install-nix-action@5d66827

Author: loafofpiecrust

.github/workflows/main.yml

@@ -77,7 +77,8 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v13
         with:
-          install_url: https://nixos-nix-install-tests.cachix.org/serve/lb41az54kzk6j12p81br4bczary7m145/install
+          nix_path: nixpkgs=channel:nixos-21.05
+          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