Move GKE temporary directories to ephemeral storage

[michaelkedar]

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 to the worker nodes to use for the working directory & temp files.