The macOS system updater often rewrites /etc/bashrc which removes the stuff added by the Nix installer, and I have to go hunting for the code every time. So, for future reference, the installer adds these lines to the end of /etc/bashrc:
# 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