Disable separate etc output for hscolour.

This is not needed, since `hscolour` has no output to `/etc`, and it
breaks certain old builds (see https://hydra.nixos.org/build/57925511)
because the older cabal used does not understand this flag.

pkgs/development/haskell-modules/make-package-set.nix

@@ -44,6 +44,7 @@ let
       isLibrary = false;
       doHaddock = false;
       hyperlinkSource = false;      # Avoid depending on hscolour for this build.
+      enableSeparateEtcOutput = false; # The flag to support this is missing in old versions of cabal.
       postFixup = "rm -rf $out/lib $out/share $out/nix-support";
     });
     cpphs = overrideCabal (self.cpphs.overrideScope (self: super: {