Switch to cachix/install-nix-action@v20

Fixes the installation issue with the latest Nix.

Also revert the pinning to nix-2.13 since it's not needed any more.

.github/workflows/ci.yml

@@ -19,9 +19,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
-    - uses: cachix/install-nix-action@v19
-      with:
-        install_url: https://releases.nixos.org/nix/nix-2.13.3/install
+    - uses: cachix/install-nix-action@v20
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
     - uses: cachix/cachix-action@v12
       if: needs.check_secrets.outputs.cachix == 'true'