nixUnstable: fix cross by disabling documentation

[alyssais]

Motivation for this change

The documentation for Nix HEAD now requires running the generated Nix
binary, so we can't cross-build it. :(

NixOS/nix#4622

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[SuperSandro2000]

@alyssais

nix-build -A pkgsCross.aarch64-multiplatform.nixUnstableerror: Package ‘nix-2.4pre20210601_5985b8b’ in ~/.cache/nixpkgs-review/pr-125772/nixpkgs/pkgs/tools/package-management/nix/default.nix:160 has invalid meta.outputsT
oInstall, refusing to evaluate.
       The package nix-2.4pre20210601_5985b8b has set meta.outputsToInstall to: out, man

       however nix-2.4pre20210601_5985b8b only has the outputs: out, dev

       and is missing the following ouputs:

         - man
(use '--show-trace' to show detailed location information)

[alyssais]

Yikes, kind of unfortunate that there's no way to catch that using stable Nix...

[SuperSandro2000]

This is a semi-automatic executed nixpkgs-review with nixpkgs-review-checks extension. It is checked by a human on a best effort basis and does not build all packages (e.g. lumo, tensorflow or pytorch).

Result of nixpkgs-review pr 125772 run on x86_64-linux 1

5 packages built:

• hydra-unstable
• nix-direnv
• nix-update
• nixUnstable
• nixpkgs-review

[alyssais]

It is possible to catch it, turns out. It's a Nixpkgs feature, not a Nix one. I just needed to turn on checkMeta. :)