test: set benchmark-child-process flaky on windows

sequential/test-benchmark-child-process is still failing sometimes flaky
on Windows in CI. Mark it as flaky in sequential.status until it gets
sorted.

PR-URL: https://github.com/nodejs/node/pull/12561
Ref: https://github.com/nodejs/node/issues/12560


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>