Override and cross-compile a package without rebulding dependencies

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Hello, I am trying to create a flake that would provide ffmpeg libraries compiled with Rockchip MPP library.

For that i defined following variables:

pkgs = import nixpkgs {
  system = system;
};

pkgsCross = import nixpkgs {
  system = system;
  crossSystem = "aarch64-linux";
  overlays = [
    self.overlays.${system}.rockchip
  ];
};

and rockchip overlay:

rockchip = final: prev: {
  rkmpp = prev.stdenv.mkDerivation rec {
    pname = "rkmpp";
    version = rkmppVersion;
    src = prev.fetchFromGitHub {
      owner = "rockchip-linux";
      repo = "mpp";
      rev = "${version}";
      sha256 = "sha256-X2PeBAdQgZjh8U8BJcHfwv0kdmKUL7wB5kZbAn2EqVE=";
    };
    nativeBuildInputs = with prev.buildPackages; [ gcc cmake ];
    cmakeFlags = [
      "-DCMAKE_INSTALL_INCLUDEDIR=include"
      "-DCMAKE_INSTALL_LIBDIR=lib"
    ];
  };
};

How to override and cross-compile ffmpeg package in such a way, that it would add rkmpp package to buildInputs of ffmpeg and --enable-rkmpp to configureFlags and would not trigger rebuild of all ffmpeg dependencies?

I’ve read Lazy cross-compiling part of Cross Compiling page on wiki, but i don’t know how to apply it here.