[bug] Lingering processes after eval run completion

[hvaara]

@terryyz reported on Slack:

One issue I haven't figured out is that there are always some live processes when the execution finishes, even though they are supposed to be killed.
Any bigcodebench.evaluate commands for sanitized outputs will result in such behaviors, regardless of whether it's in the docker or outside
The example sanitized outputs can be found here: https://github.com/bigcode-project/bigcodebench/releases/tag/v0.1.5
It's a generic issue, but I haven't found the cause.
I fixed a bit of the customized sandbox to kill the child processes when it finishes (https://github.com/bigcode-project/bigcodebench/blob/main/bigcodebench/eval/utils.py), but it didn't resolve the issue