Minor fix in query_bench set_nested_value benchmark: (#630)
Minor fix in query_bench set_nested_value benchmark: (#630)

`rebuild` shallow clones the message and returns an updated message, so
return value should be used.

Thanks to @sigurdm for noticing this.
dart-lang/dart-protobuf,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/protobuf,google/protobuf.dart,google/protobuf.dart,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/protobuf