Try bumping install-nix action

Author: michaelpj

.github/workflows/nix.yaml

@@ -15,7 +15,7 @@ jobs:
       - uses: actions/checkout@v2
         with:
           submodules: true
-      - uses: cachix/install-nix-action@v12
+      - uses: cachix/install-nix-action@v16
         with:
           nix_path: nixpkgs=channel:nixos-unstable
       - run: nix-shell --run "cabal update && cabal build all"