default.nix

with import <nixpkgs> {};

let mkEmojicodeApp = attrs: stdenv.mkDerivation ({
  setupHook = ./setup-hook.sh;
} // attrs);
in
mkEmojicodeApp {
  name = "test";
  src = ./src;

  buildInputs = [ hello ];

  installPhase = ''
    echo $EMOJICODE_PATH
  '';

}

setup-hook.sh

addPackagePath () {
      addToSearchPath EMOJICODE_PATH $1/lib/emojicode
      EMOJICODE_PATH += $1/lib/emojicode
      echo $1
}

addEnvHooks "$targetOffset" addPackagePath