cascache: move tmp directory one level up

The CAS uses a temp directory while manipulating the cache, temporary
files can be added and removed while adding artifacts to the cache here.

Since calculation of the cache size happens in parallel to artifact
cache additions, this causes race conditions in the size calculation
job, as we end up calling `stat` on temporary files which are being
removed in parallel.

Handle this by moving the temporary directory out of the way, and avoid
considering the tmp directory when calculating the cache size