Hi I am using nix package manager together with nix-darwin on catalina when I try to install a package that uses gcc it thinks my macos version is 10.12 any ideas why? 
my shell.nix is: 
```
with import <nixpkgs> {};
stdenv.mkDerivation rec {
  name = "polynote_python-shell-mac";
  env = buildEnv { name = name; paths = buildInputs; };
  buildInputs = [
    darwin.cf-private
    darwin.apple_sdk.frameworks.CoreServices
    python38
    python38Packages.virtualenv
    python38Packages.pip
    python38Packages.jedi
    python38Packages.pyspark
  ];
  shellHook = ''
    SOURCE_DATE_EPOCH=$(date +%s)
    mkdir -p .env
    virtualenv .env
    source .env/bin/activate
    export PYTHONPATH=.env/lib/python3.8/site-packages/:$PYTHONPATH
    export MYENV=polynote_nix_python
    pip install -U pip
    pip install git+https://github.com/softinio/jep.git@support-nix-mac
  '';
}


```