Use cachix install-nix version 14.1 (compatibility with private runner).

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).  It is
reported that version 14.1 does not have this issue.

.github/workflows/ci.yaml

@@ -29,7 +29,7 @@ jobs:
         submodules: true
 
     - name: Install Nix
-      uses: cachix/install-nix-action@v16
+      uses: cachix/install-nix-action@v14.1
       with:
         nix_path: nixpkgs=channel:nixos-21.11
         install_url: https://releases.nixos.org/nix/nix-2.4/install