runtime/race: RaceChanWrongSend flake on linux-amd64

[bradfitz]

This change failed on linux-amd64-race:
See https://storage.googleapis.com/go-build-log/99e58459/linux-amd64-race_abd9d5c2.log

NoRaceChanSyncCloseRecv2                 .
NoRaceChanSyncCloseRecv3                 .
RaceChanSyncCloseSend                    .
RaceChanAsyncCloseSend                   .
RaceChanCloseClose                       .
RaceChanSendLen                          .
RaceChanRecvLen                          .
RaceChanSendSend                         .
NoRaceChanPtr                            .
RaceChanWrongSend                        FAILED
RaceChanWrongClose                       .
RaceChanSendClose                        .
RaceChanSendSelectClose                  .
RaceSelectReadWriteAsync                 .
RaceSelectReadWriteSync                  .
NoRaceSelectReadWriteAsync               .
RaceChanReadWriteAsync                   .
RaceChanReadWriteSync                    .

/cc @dvyukov

[dvyukov]

dup of #14119