automation of NIX_GHC_LIBDIR environment variable:

Hello guys, I installed ghcide which is working great, but everytime that I do a rebuild I have to run: cat $(which ghc),
then set the newly generated nix path of NIX_GHC_LIBDIR in the in it's corresponding sessionVariable of my 
home.nix configuration, is there a way to automate this process so I don't have to keep doing this manually
over and over again?

here is an issue that someone else is having that is similar to my issue:
https://github.com/hercules-ci/ghcide-nix/issues/26

also, here is my sessionVariables code section of my home.nix configuration file:

    sessionVariables = {
      EDITOR = "emacs";
      HIE_HOOGLE_DATABASE = "$(cat $(which hoogle) | sed -n -e 's|.*--database \\(.*\\.hoo\\).*|\\1|p')";
      NIX_GHC_LIBDIR = "/nix/store/mc4nx9v5fbz5dwd0kabfpxkzjd5zaz8c-ghc-8.6.5-with-packages/lib/ghc-8.6.5";
    };

here are are the commands that i have tried to set NIX_GHC_LIBDIR to:

NIX_GHC_LIBDIR="${pkgs.ghc}/lib/ghc-${pkgs.ghc.version}"

${pkgs.ghc}/lib/ghc-*

NIX_GHC_LIBDIR="${pkgs.ghc.outPath}/lib/ghc-${pkgs.ghc.version}";

here is my home.nix file configuration: https://hastebin.com/ohenivaway.bash