ref: move prepared files to ~/.cache dir

[jordanrfrazier]

Moves the prepared files directory to ~/.cache/kaskada/.

The other alternative is to, for now, create a temporary directory that cleans up after use. While the ~/.cache is generally used for application-specific files (which is what the prepared files are), we aren't utilizing them and won't until we have state. But, if it's not a large concern to accumulate prepared files in the cache, we might as well start with it here in anticipation of using them.

Closes #779

[bjchambers]

Until we start re-using them, we really should delete them. Otherwise, every run of the test creates files that build up over time, eventually leading to out of disk space. And now it's in a hidden cache directory.

I think:

1. Preparing to a temporary or cache directory and deleting it is the way to go for now.
2. Soon, we want to re-enable prepare to S3 (more storage) and then (possibly) use the local cache directory for things like the rocksdb and a cache of downloaded files... at which point we should manage it like a cache and limit the size, etc.