It looks like NixPkgs channels have moved. Fixing this URL allows using nixos-20.09, for instance.
Use a NixOS / NixPkgs release that's actually being supported and regularly updated.
This gets rid of the package called "server" and instead moves everything into the project root, such that Go actually builds us a binary called `nixery`. This is the first step towards factoring out CLI-based functionality for Nixery.
