Nevermind, figured I’d see where that code was coming from, and stumbled across others with the same issue. Looks like it’s some recent incompatibility between home-manager unstable and nixpkgs stable: Nixos-rebuild fails with current configuration - #7 by klt
Just switch to home-manager’s stable branch, I guess? They are supposed to always match (i.e. use stable on both, or unstable on both, but never a mix), but it won’t always result in problems if they don’t.