This is a hacky appimage wrapper nixpkg for PureRef to integrate it into NixOS better. As you may know, it's donationware, so the author encourages a donation and this means there's no way around manually downloading the program. So as a prerequisite, you need to download PureRef separately and slap it in the same folder as the Nix file, or alternatively, change the path to an absolute one, e.g.
src = /home/you/Downloads/PureRef-1.11.1_x64.Appimage;
Ideally this would use the actual Nixpkg appimage build tooling... but that's effort, and I just want to get shit done right now and this works fine.