Move GKE temporary directories to ephemeral storage

Currently, our worker/importer/exporter use the attached persistent SSDs to store temporary data such as git repositories, which can cause problems if the files are not deleted before the pod exits.

#1666 somewhat addressed this by deleting the tmp folder on startup, but we shouldn't actually be storing these files on the that SSD anyway.

We should add an [ephemeral ssd](https://cloud.google.com/kubernetes-engine/docs/concepts/local-ssd#api) to the worker nodes to use for the working directory & temp files.