Drupal 8 Sync Files fails due to files/php cache folder owned by www-data

Hi!

We cannot fire a sync files task reliably with drupal 8 sites because there is a new "php" folder that is created with PHP caches for things like twig.

This folder is owned by www-data so it breaks when trying to use the "Sync Data" task.

Is there a way that this task skips certain folders? We should add sites/%/files/php to the list of skipped folders, both for hosting _sync and aegir backups.