allow access to the nixpkgs haskell utility library

I almost used it to narrow these two dependencies to library-only but it turns
out to not make much difference so it doesn't seem worth the extra complexity
now.

However, having access to the library might prove useful in the future.

flake.nix

@@ -34,7 +34,9 @@
         src = ./.;
         compilerVersion = ghcVersion;
         packageName = "gbs-downloader";
-        hsPkgsOverrides = import ./nix/haskell-packages.nix;
+        hsPkgsOverrides = import ./nix/haskell-packages.nix {
+          haskellLib = pkgs.haskell.lib;
+        };
       };
     in {
       checks = hslib.checks {};

nix/haskell-packages.nix

-hfinal: hprev: {
+{haskellLib}: hfinal: hprev: {
   tahoe-chk = hfinal.callPackage ./tahoe-chk.nix {};
   tahoe-great-black-swamp = hfinal.callPackage ./tahoe-great-black-swamp.nix {};
 }