Z3 dependency in NixOS

[andrew222651]

In case anyone else tries to run CrossHair on NixOS, Z3 (as automatically installed by pip as a dependency of CrossHair) wouldn't work until I added the output of nix --extra-experimental-features nix-command eval --impure --raw --expr 'with import <nixpkgs> {}; stdenv.cc.cc.lib', with /lib added at the end, to the LD_LIBRARY_PATH env var. Maybe there's a way of using the Z3 Nix package.