Flake: how make nixpkgs' self follow another input's nixpkgs?

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{
  nixConfig.substituters = [ "https://hydra.iohk.io" ]; # only pinned nixpkgs's builds are cache hits!!!
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/master";
  inputs.haskell-nix.url = "github:input-output-hk/haskell.nix?ref=flake-fixes";

  # How make so that inputs.nixpkgs **is** inputs.haskell-nix.nixpkgs?
  # like so? 
  inputs.self.nixpkgs.follow = "haskell-nix";
  # or like so?
  inputs.nixpkgs.self.follow = "haskell-nix";
  outputs = { self, nixpkgs, haskell-nix }: {
  };
}

The goal is to increase cache hits when building overlays.

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There was another thread with the same question a few days ago. See:

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