fix: handle SIGTERM correctly on Fargate

[hassy]

Description

Fix handling of SIGTERM:

• Exit with appropriate exit code (143) from the run command
• On Fargate (e.g. when stopped early by Fargate Spot, or via explicit task stop, e.g. through aws ecs stop-task) - replace the use of wait with timeout. wait cannot be used for processes that run in a subshell - https://github.com/artilleryio/artillery/blob/27fae398ce0d6bab5f4226e361a9de6d4d8bec2a/packages/artillery/lib/platform/aws-ecs/worker/loadgen-worker#L277

Pre-merge checklist

This is for use by the Artillery team. Please leave this in if you're contributing to Artillery.

• [x] Does this require an update to the docs? No
• [ ] Does this require a changelog entry? Yes

[hassy]

Existing E2E tests passing on Fargate. Adding an E2E test for the error condition needs an easy way to get the task ARNs for a running test, so leaving for later. Tested by stopping a task in a running test manually and everything looks good.