Manually add
if [[ -f '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]]; then
source '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
export NIX_PATH="$HOME/.nix-defexpr"
fi
to ~/.zprofile
.
Undo the changes to /etc/zshrc