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.
2872e10333