[CI] fix 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).

.github/workflows/ci.yaml

@@ -12,6 +12,7 @@ jobs:
       LC_ALL: en_US.UTF-8
       TZ: America/Los_Angeles
       CI_TEST_LEVEL: "2"
+      USER: root
 
     runs-on: ${{ matrix.os }}
     strategy:
@@ -29,7 +30,7 @@ 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