runsc: fix pid file race condition in exec detach mode.
runsc: fix pid file race condition in exec detach mode.

PiperOrigin-RevId: 214700295
Change-Id: I73d8490572eebe5da584af91914650d1953aeb91

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor