rsync should be required in nix-manual meson.build

[VaiTon]

Description of the problem

When building the nix-manual meson subproject a custom target is defined using a shell script that invokes rsync. Despite that, rsync is never required by meson and configuration still succeeds if no rsync is on the system.

https://github.com/NixOS/nix/blob/86767349d5fc09326801db96fa427511862ddf54/doc/manual/meson.build#L87

Suggested solution

Add a find_program('rsync', required: true, native: true) to the meson.build

[roberth]

Triaged in Nix meeting today:

Seems like this could be implemented.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2025-06-11-nix-team-meeting-minutes-231/65543/1