Broken as of nixos-22.05

[liesnikov]

I'm guessing the way the overlays work changed at some point, since at the moment I can't build example.nix with pinned nixpkgs:

let
  defpkgs = builtins.fetchTarball {
    # Descriptive name to make the store path easier to identify
    name = "nixos-22.05.1271.babb041b716";
    url = "https://releases.nixos.org/nixos/22.05-small/nixos-22.05.1271.babb041b716/nixexprs.tar.xz";
    # Hash obtained using `nix-prefetch-url --unpack <url>`
    sha256 = "0g8vwni83zn6kgkczrm5vwmyhl473rrs9d4k4hn5gfbgfsyv7ls8";
  };
  plugin-overlay-git = builtins.fetchGit
   { url = https://github.com/mpickering/haskell-nix-plugin.git;}  ;
  plugin-overlay = import "${plugin-overlay-git}/overlay.nix";
  nixpkgs = import defpkgs { overlays = [plugin-overlay]; };

  hl = nixpkgs.haskell.lib;
  hp = nixpkgs.haskellPackages;
in
  (hp.withPlugin(plugs: ps: hl.addPlugin plugs.dump-core ps.either)).DumpCore

This gives me an error

error: attribute 'withPlugin' missing

       at /home/user/example/nix/default.nix:17:4:

           16| in
           17|   (hp.withPlugin(plugs: ps: hl.addPlugin plugs.dump-core ps.either)).DumpCore
             |    ^

FWIW this issue is more to document the breakage than an expectation for you to fix it, maybe I'll find inspiration to look into it myself later.