clean up work and artifact dirs on task claim?

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.

Start by locating the task-claim path and the startup code that creates the work and artifact directories. Compare the options of always recreating them or making cleanup conditional on a new preference. Done means leftover files from a prior task or manual signer testing cannot affect the next claimed task, with behavior checked on both Docker workers and Mac signers.