This reverts commit 6a756af3e7.
Currently zshenv by default only set fpath and HELPDIR without exporting them.
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 __NIXOS_SET_ENVIRONMENT_DONE. Hence any modification
done by the parent shell should persist as long as __NIXOS_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.
Fixes#80437
55819e6c86