nix file that provides a way to inject zsh dotfiles inside of a `shellHook`

example.nix

{ pkgs ? import <nixpkgs> {}
, zdotdir ? import (builtins.fetchurl {
    url = "https://gist.githubusercontent.com/chisui/bba90fccc930f614743dc259fbadae6d/raw/4108222addc1d646c1b0a6d12130083e2219ad28/zdotdir.nix";
  }) { inherit pkgs; }
}:
pkgs.stdenv.mkDerivation {
  name = "withZshEnv";
  shellHook = zdotdir {
    zshenv = ''
      alias taco=echo\ "tacos!";
    '';
  };
}

zdotdir.nix

{ pkgs ? import <nixpkgs> {}
}:
{ zshenv ? ""
, zshrc  ? "" 
}:
let
  mkZshDotFile = fileName: content: builtins.toFile "custom.${fileName}" ''
    if [ -z $\{ZDOTDIR_PARENT\} ];
    then
      DOTFILE=$ZDOTDIR_PARENT/.${fileName};
    else
      DOTFILE=$HOME/.${fileName};
    fi;
    if [ -f $DOTFILE ];
    then
      source $DOTFILE;
    fi;
    ${content}
  '';
  zDotDir = pkgs.stdenv.mkDerivation {
    name = "ZDOTDIR";
    phases = ["installPhase"];
    installPhase = ''
      mkdir $out
      cp ${mkZshDotFile "zshenv" zshenv} $out/.zshenv
      cp ${mkZshDotFile "zshrc" zshrc} $out/.zshrc
    '';
  };
in ''
  if [[ -v ZDOTDIR ]];
  then
    export ZDOTDIR_PARENT=$ZDOTDIR;
  fi
  export ZDOTDIR=${zDotDir};
''