nixos/manual: apply `cfg.warningsAreErrors` to nix eval as well

This change makes `baseOptionsJSON` (`lazy-option.json`) take
our existing `documentation.nixos.options.warningsAreErrors` option into
account to then set the `NIX_ABORT_ON_WARN` environment variable within
its build sandbox accordingly.

Given `documentation.nixos.options.warningsAreErrors` default to true,
our nixpkgs CI check that builds the NixOS manual for each PR, will now
fail when something raises a warning, e.g. as part of the `lib.mdDoc`
deprecation.

This should prevent new PRs with lib.mdDoc from getting merged.

Previously, the manual would build successfully, even if an eval warning
was raised.

nixos/modules/misc/documentation.nix

@@ -101,6 +101,7 @@ let
           libPath = filter (pkgs.path + "/lib");
           pkgsLibPath = filter (pkgs.path + "/pkgs/pkgs-lib");
           nixosPath = filter (pkgs.path + "/nixos");
+          NIX_ABORT_ON_WARN = warningsAreErrors;
           modules =
             "[ "
             + concatMapStringsSep " " (p: ''"${removePrefix "${modulesPath}/" (toString p)}"'') docModules.lazy