This reverts commit 909d8cb293.
This messes up PATH priority since /etc/profile gets sourced AFTER
/etc/zshenv and it sets the system paths so
$HOME/.nix-profile/bin:/nix/var/nix/profiles/default/bin is behind
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin. See discussion in
https://github.com/NixOS/nix/issues/4169.