clean up work and artifact dirs on task claim? #487
Description
Historically, we've created the work and artifact dirs on scriptworker startup, and assume the directories exist and are empty on task claim. This is efficient on docker instances where we don't expect anyone or anything to write to these directories.
On the mac signers, we sometimes ssh in and test something. We currently need to make sure we clean up after ourselves, or the next task that runs can be affected by leftover cruft in the work or artifact dirs. I'm happy with either deleting/recreating these directories on task claim everywhere; or creating a new pref and deleting/recreating if the pref is set.