Drush cache clears can cause Cloud Hooks failure

Issue

When a cloud hook is running a drush cr while another job is executing a drush command that is using drush cache, an error can occur:

Started drupal cache rebuild for *** [success]
Cache rebuild complete. 

In Filesystem.php line 181: Failed to remove directory "/home/***/.drush/cache/factory": rmdir(/home/****/.drush/cache/factory): Directory not empty

Resolution

The solution is to isolate the cache directory between processes using the DRUSH_PATHS_CACHE_DIRECTORY variable