[CI] remove cachix install-nix

There appears to be some unknowns regarding ensuring nix is installed,
but not overwritten (see
cachix/install-nix-action#98 and
https://github.com/cachix/install-nix-actions/pull/100/files).

Ultimately however, this step is not needed for the current Galois
internal runners that are already nix-based configurations.

.github/workflows/ci.yaml

@@ -28,12 +28,6 @@ jobs:
       with:
         submodules: true
 
-    - name: Install Nix
-      uses: cachix/install-nix-action@v16
-      with:
-        nix_path: nixpkgs=channel:nixos-21.11
-        install_url: https://releases.nixos.org/nix/nix-2.4/install
-
     - uses: cachix/cachix-action@v10
       with:
         name: galois