nixos-option: remove if `nix.package` is unstable

This was done in a0a61c3e34, but users
typically install `nixUnstable` via `nix.package`, not by overlaying
`pkgs.nix` directly.

nixos/modules/installer/tools/tools.nix

@@ -40,7 +40,7 @@ let
   };
 
   nixos-option =
-    if lib.versionAtLeast (lib.getVersion pkgs.nix) "2.4pre"
+    if lib.versionAtLeast (lib.getVersion config.nix.package) "2.4pre"
     then null
     else pkgs.callPackage ./nixos-option { };