Packaging an existing texlive package

1

I want to update a texlive package. The package is already in nix packages but not upto date. So in home manager i added the updated package by using the texlive.combine function and included the package by pulling it directly from github source. Now after building the derivation when i do kpsewhich package.sty it seems to be using the old package. That means I am not overriding it properly .

 {pkgs,fetchFromGitHub,...}:
pkgs.stdenvNoCC.mkDerivation rec{
  name = "stex";
  src = fetchFromGitHub {
    owner = "slatex";
    repo = "sTeX";
    rev = "stex4";
    sha256 = "ZPAplgopAdEnPzguFDA2qSrpMgmTcxvxY8ciT8GCvjQ=";
  };
  sourceRoot = "${src.name}/tex";
  installPhase = "cp -r $src $out";
  passthru.tlType = "run";
}

This above code is imported in home-manager derivation. then my home-manager looks something like this

some code here .....
let pkgs-unstable = import <nixpkgs-unstable> {};
stex = pkgs.callPackage ./modules/stex {}; 
in 
{
..........
home.packages = [
  (pkgs-unstable.texlive.combine {
      inherit (pkgs-unstable.texlive) scheme-full;
      stex = {pkgs = [stex];};
    })
];
............
};

so this is the code i am using the stex package should be overriden in the scheme-full
but its not working my texlive has the old stex.sty files