Apple Borked my Nix!
Ok, so you've had nix (home-manager) working fine. Then Apple tells you it's time to update.
Ok. Reboot. Oops. It has now broken your Nix setup. Here's some stuff to work through. YMMV.
Note: This is what worked for me, who was just using nix + home-manager. The upgrade that I last did that caused all these issues was 12.3.X > 12.4