make inclusion of HIE overlay conditional as well

default.nix

@@ -14,13 +14,13 @@ let
       name = "nixpkgs-pinned";
       url = https://github.com/NixOS/nixpkgs/;
       ref = "refs/heads/release-20.03";
-      rev = "de3780b937d2984f9b5e20d191f23be4f857b3aa";
+      rev = "faf5bdea5d9f0f9de26deaa7e864cdcd3b15b4e8";
     }) {
       # Pass no config for purity
           config = {};
-              overlays = [
+              overlays = if withHIE then [
                       (import all-hies {}).overlay
-                          ];
+                          ] else [];
     };
   hp = pkgs.haskell.packages."${compiler}";
   src = pkgs.nix-gitignore.gitignoreSource [] ./.;