[bitrisescript] Attempt to cancel Bitrise builds when Task is pre-empted or cancelled

[ahal]

When a bitrisescript task is cancelled or has an infrastructure related error (e.g is pre-empted), we should attempt to cancel the in-progress Bitrise builds as well as they won't be useful anymore.

I think we at least need to handle SIGTERM in script.py, then cancel any in-progress futures, as well as add done callbacks to them to trigger the Bitrise API to abort their associated build.

I'm not 100% sure if a user cancelling the task will send SIGTERM or not..

[jcristau]

AIUI, if the task gets canceled, we'll get a 409 the next time scriptworker calls reclaimTask, which should mean the task gets SIGTERM via https://github.com/mozilla-releng/scriptworker/blob/cd0aac0510f69386b28a375656b5cc070dc2fb7f/src/scriptworker/task.py#L718