nixos/man-db: trigger mandb generation by multi-user.target (#495430)

            BindReadOnlyPaths = [ "/dev/null:/etc/man_db.conf" ]; # mandb will still read /etc/man_db.conf if it exists, even when setting -C path/to/config.conf

            ProtectSystem = "strict";

          };

          wantedBy = [ "default.target" ];

          wantedBy = [ "multi-user.target" ];

        };

      })

    ]