I’m worried your store is corrupted somehow. Can you try nix-store --verify --check-contents --repair (or nix store verify --all --no-trust --repair for newstyle CLI and output log)? This will verify all the data in the nix store
Hm. Well I tried building it with nix build github:eureka-cpu/dotfiles#nixosConfigurations.critter-tank.config.system.build.toplevel --update-input nixpkgs --no-write-lock-file and it succeeded. That’s why I thought it might be local corruption or something.
I had a similar issue some time back and it was due to an issue/bug with nix, I had to build the system with a different version of it (e.g. on a different system).