When I build socrates using `sudo nixos-rebuild [...] switch`, my
`nixos-config` (i.e. <briefcase/nixos/socrates/default.nix>) is a simple Nix
anonymous function. Typically readTree populates my pkgs, briefcase, depot
function parameters with <nixpkgs>, <briefcase>, <depot>, but `nixos-rebuild` is
unaware of `readTree`.
For now I'm manually importing these dependencies, and I'm leaving a TODO to
reconsider switching to the `{ pkgs, briefcase, ... }` style when I better
understand NixOS.