Maybe the upgrade reset your /etc/zshrc. Is Nix mentioned anywhere there?
If not, look for the relevant profile script, I think in in /nix/var/nix/profiles/default/etc/profile.d. I think it may be called nix-daemon.sh. Normally on macOS, /etc/bashrc and /etc/zshrc check if that file exists in an if block, then source it if it does.
The other thing you might have to set back up if it got nuked is the launchd entry that automatically starts the Nix daemon
# Nix
if [ -e '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh' ]; then
. '/nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh'
fi
# End Nix
to the end of my /etc/zshrc and that seemed to do the trick! Thanks @abathur!