Procedurally, I guess? I don’t seem to have an init.el file. I maintain a .emacs file in my home folder, just like I did on Ubuntu before NixOS. I tried adding the line you suggested to it, and still get the same error.
According to @jethrokuan, the problem is (if I understand him) that org-roam recently changed and will require an override until nixpkgs catches up. I’m currently trying to understand overlays; maybe once I do I can act on his suggestion.