using nickel flake in home-manager always leads to rebuild

[suimong]

As stated in the title, though I'm still not too comfortable with Nix stuff so it's likely me doing something wrong or probably an issue on the nix/home-manager side rather than a nickel one.

The situation is that when used like nix run github:tweag/nickel/<tag> or nix shell github:tweag/nickel/<tag> the binary cache works and no compilation is ever needed. However if I use github:tweag/nickel/<tag> as my nixos configuration flake input, and reference the input inside the home-manager module through extraSpecialArgs, then regardless of which <tag> I choose, a full rust compilation was always performed (on the first time this <tag> is evaluated).

Below is an example config for my nix/home-manager setup on Ubuntu:

# flake.nix
{
  description = "Home Manager configuration";

  inputs = rec {
    # Specify the source of Home Manager and Nixpkgs.
    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixos-unstable";
    nixpkgs = nixpkgs-unstable;
    home-manager = {
      url = "github:nix-community/home-manager";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    nickel = {
      url = "github:tweag/nickel/1.5.0";
      # inputs.nixpkgs.follows = "nixpkgs";      <-- Whether this line is commented or not has no effect to using binary cache.
    };
  };

    outputs = { nixpkgs, nixpkgs-unstable, home-manager, nickel, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        config.allowUnfree = true;
      };
      pkgs-unstable = import nixpkgs {
        inherit system;
        config.allowUnfree = true;
      };
    in {
      homeConfigurations."ben" = home-manager.lib.homeManagerConfiguration {
        inherit pkgs;

        # Specify your home configuration modules here, for example,
        # the path to your home.nix.
        modules = [ ./home.nix ];
        extraSpecialArgs = {
          inherit nickel system pkgs-unstable;
        };
      };
    };
}

# home.nix
{ config, pkgs, pkgs-unstable, system, nickel, ... }:
{
  ...
  home.packages = [
    # nickel.packages.${system}.nickel-lang-cli   <-- also tried to specify the package output, but still recompiles.
    nickel.packages.${system}.default
  ];
}

Any guidance is appreciated.