Currently zshenv by default only sets fpath without exporting it. A
parent shell would also not set those variables usually as they are
shell local.
It also sources a file called set-environment but this is protected by
an environment variable called __NIX_DARWIN_SET_ENVIRONMENT_DONE. Hence
any modification done by the parent shell should persist as long as
__NIX_DARWIN_SET_ENVIRONMENT_DONE is not unset.
This behavior deviates from what we do in bashrc and breaks common
setups such as tmux/mosh or screen.
NixOS/nixpkgs@55819e6c86
7e6c548eef