Mark single Haskell package as not broken in flakes?

I’m trying to get a Haskell project to build with flakes, so I added

packages.backend = pkgs.haskellPackages.developPackage {
  root = ./backend;

to my flake.nix.

When I build this I get the error Package ‘servant-elm-0.7.3’ in /nix/store/dvjl75ai4xf2jb11ihhlcg4ly1wlm7j2-source/pkgs/development/haskell-modules/hackage-packages.nix:260075 is marked as broken, refusing to evaluate.

The error also suggest to set allowBroken = true; but I’d rather just override this one package’s broken flag instead of allowing all broken packages.

So I tried adding an overlay (first time doing this as well):

unbreakServantElmOverlay = final: prev: {
  haskellPackages = prev.haskellPackages.extend (
    hfinal: hprev: {
      servant-elm = hprev.servant-elm.overrideAttribute { broken = false; };
pkgs = nixpkgs.legacyPackages.${system}.extend unbreakServantElmOverlay;

And when I build then I still get the same error, saying that servant-elm is broken.

What am I doing wrong?