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This is primarily to ensure that -I nixpkgs=https://nixos.org/channels/nixos-unstable/nixexprs.tar.xz and -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-unstable.tar.gz and -I nixpkgs=https://nixos.org/channels/nixpkgs-unstable/nixexprs.tar.xz behave consistently. It also allows installing packages via "nix-env -iA nixos.<pkg>" rather than "nixos.pkgs.<pkg>". It would be even better to allow "nixpkgs.<pkg>", but that requires a change to nix-channel. Fixes #7659.
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.