Self-hosted agents can shutdown without flushing pending logs

[betelgeuse]

Scenario:

• One uses self-hosted agents
• One configures a shutdown hook to bring down the machine fast as described in the documentation https://docs.semaphoreci.com/ci-cd-environment/configure-self-hosted-agent/#shutdown-hook-path

Actual results:

• The log pushing runs asynchronously
• Fast shutdown can mean that the log upload never happens

Expected results:

• The agent flushes the logs before doing shutdown

[lucianoliberti]

@betelgeuse thanks, we'll add it to the list to be analyzed.