Upgrade to cachix-action@v8

Adds support for auth tokens

Author: pepeiborra

.github/workflows/nix.yml

@@ -18,7 +18,7 @@ jobs:
     - uses: cachix/install-nix-action@v12
       with:
         nix_path: nixpkgs=channel:nixos-20.03
-    - uses: cachix/cachix-action@v7
+    - uses: cachix/cachix-action@v8
       with:
         name: haskell-language-server
         extraPullNames: haskell-ghcide