README: update install-nix-action

As of v15, you don't need the janky install_url or install_options stuff
anymore.

README.md

@@ -22,12 +22,9 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v2
       - name: Install Nix
-        uses: cachix/install-nix-action@v14
+        uses: cachix/install-nix-action@v16
         with:
-          install_url: https://nixos-nix-install-tests.cachix.org/serve/vij683ly7sl95nnhb67bdjjfabclr85m/install
-          install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
           extra_nix_config: |
-            experimental-features = nix-command flakes
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: Update flake.lock
         uses: DeterminateSystems/update-flake-lock@v3