9df7efe0c6
Before, we very carefully unapplied and reapplied `set -u` so the rest of Nixpkgs could continue to not fail on undefined variables. Let's rip off the band-aid.
To build the manual, you need Nix installed on your system (no need
for NixOS). To install Nix, follow the instructions at
https://nixos.org/nix/download.html
When you have Nix on your system, in the root directory of the project
(i.e., `nixpkgs`), run:
nix-build nixos/release.nix -A manual.x86_64-linux
When this command successfully finishes, it will tell you where the
manual got generated.