Fix flakiness in CI

[allada]

This is a tracking bug to track a flakey test in CI.

Flaky tests:

• [ ] experimental_precondition_script_fails
• [ ] entrypoint_does_invoke_if_set
• [ ] entrypoint_injects_properties
• [ ] entrypoint_sends_timeout_via_side_channel

Right now we are just sleeping a little before the test matters.

Example failure: https://github.com/TraceMachina/nativelink/actions/runs/7268320030/job/19803924206