Use nixos-unstable in CI

Author: berberman

.github/workflows/nix.yml

@@ -27,7 +27,7 @@ jobs:
         submodules: true
     - uses: cachix/install-nix-action@v12
       with:
-        nix_path: nixpkgs=channel:nixos-20.03
+        nix_path: nixpkgs=channel:nixos-unstable
     - uses: cachix/cachix-action@v8
       with:
         name: haskell-language-server