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.

query_benchmark/bin/set_nested_value.dart

@@ -20,7 +20,7 @@ main() {
       title: 'protobuf_decode',
       duration: measure(() => a.rebuild((f0.A0 a0Builder) {
             a0Builder.a4.last = a0Builder.a4.last.rebuild((f2.A1 a1builder) {
-              a1builder.a378.rebuild(
+              a1builder.a378 = a1builder.a378.rebuild(
                   (f19.A220 a220builder) => a220builder.a234 = 'new_value');
             });
           })),