fix: use nixpkgs-unstable instead of nixos-unstable

As of 23:30 UTC, 2025-06-22, it seems that `nix` does not build on macOS on the
nixos-unstable channel. So the difference matters, at least on occasion.

.github/workflows/review.yml

@@ -123,7 +123,7 @@ jobs:
             pkgs+=(cachix)
           fi
           args=()
-          for pkg in ${pkgs[@]}; do args+=("github:nixos/nixpkgs/nixos-unstable#$pkg"); done
+          for pkg in ${pkgs[@]}; do args+=("github:nixos/nixpkgs/nixpkgs-unstable#$pkg"); done
           nix profile install ${args[@]}
 
       - name: clone nixpkgs