buildkite: deflake itimer test
buildkite: deflake itimer test

The self-admittedly arbitrary threshold of 20% of alarms being sent to the main
thread was being breached. I saw a run of of ~20-23% [1]. We should allow for a
little more breathing room.

1 - https://buildkite.com/gvisor/pipeline/builds/7400

PiperOrigin-RevId: 386503482

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