Sometimes build nix-manual when cross compiling

[Jayman2000]

Motivation

At the moment, the nix-manual Meson subproject won’t be built if you’re cross compiling. This pull request makes it so that the nix-manual Meson subproject will be built if you’re cross compiling Nix and the build system is capable of executing host system binaries. (See the commit message for additional details).

Context

The main motivation behind this change is to deal with this Nixpkgs issue. Building pkgs.nixStatic counts as cross compiling Nix, and pkgs.nixStatic is supposed to produce a man output. Building pkgs.nixStatic currently fails because it isn’t actually producing a man output. That issue will go away once this pull request gets backported to Nix 2.28.x.

You can verify that this pull request fixes that Nixpkgs issue by doing the following:

1. Create a file named nix-static.nix that contains this Nix expression:

   # This file is dedicated to the public domain using 🅭🄍1.0:
   # <https://creativecommons.org/publicdomain/zero/1.0>.
   let
     nixpkgsCommit = "17f6bd177404d6d43017595c5264756764444ab8";
     nixpkgsTarball = builtins.fetchTarball {
       url = "https://github.com/NixOS/nixpkgs/archive/${nixpkgsCommit}.tar.gz";
       sha256 = "1nwqbj0bmhdqf34d0y6qbpnwqa9bkhlqd03ffv60yami7fppnyb6";
     };
     pkgs = import nixpkgsTarball { };
   in
   {
     upstreamVersion = pkgs.nixStatic;
     versionFromThisPR = pkgs.nixStatic.overrideAttrs (
       finalAttrs: previousAttrs: {
         src = pkgs.fetchFromGitHub {
           owner = "Jayman2000";
           repo = "nix-pr";
           rev = "5e407e6abb5738f72b8b5aeeaa003728fe584c2f";
           hash = "sha256-s9lK09RpKklka/xDwPhmmUge19oR/zYp9lfuJkb+XHo=";
         };
       }
     );
   }

2. Build that expression by running this command:

   nix-build --keep-going <path to nix-static.nix>

3. Make sure that the build fails with exactly one error. The upstreamVersion attribute should fail to build with an error about the builder not creating a man output. The versionFromThisPR attribute should build successfully.

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[xokdvium]

@Jayman2000, thanks! The fix makes sense. It's now clear why this affected only the monolithic build of 2.28 - the top-level meson file isn't used in componentized builds.

[xokdvium]

LGTM