Calculating the cache size on the child_process_data() is racy, as this
runs in the main process which does not obtain exclusive access to the
cache resource.

This calculation also seems unnecessary, as it will triggered when
BuildQueue.done() is called.

The tmp directory is filled when adding an artifact with temporary
files. This causes problems on calculate_cache_size() because we could
try to a attemp to do a stat() on a temporary file that meanwhile has
been reaped.

Handle this by not considering the tmp directory when calculating the
cache size.

TODO:
* Uncomment when clausule.
* Remove debug information.