nixos/doc: Split `mkdir` mode into `chmod` command for clarity

        path  = [ pkgs.su ];

        script =

          ''

            mkdir -m 0755 -p $(dirname ${toString cfg.output})

            mkdir -p $(dirname ${toString cfg.output})

            chmod 0755 $(dirname ${toString cfg.output})

            exec updatedb \

              --localuser=${cfg.localuser} \

              ${optionalString (!cfg.includeStore) "--prunepaths='/nix/store'"} \