[DEVOPS-1120] Use nix-tools overlays as well as modules

Author: rvl

nix/pkgs.nix

@@ -49,6 +49,9 @@ let
   pkgSet = haskell.mkNewPkgSet {
     inherit pkgs;
     pkg-def = stackage.${stack-pkgs.resolver};
+    pkg-def-overlays = [
+      stack-pkgs.overlay
+    ];
     modules = [
      # stack-pkgs.module ## seems to not exist
      ];