In rare case, current hostmaster detection throws false positives when trying to write drushrc.php file

I've got a unique server that has a separate hostmaster-ish site in a non-standard location.

I'm not sure how this happened, as it was working fine for a while.

Running verify tasks on some of my Drupal 8 sites fails because it's trying to write Provision_Config_Drushrc_Aegir(), when it should only do that on verify of @hostmaster.

Debugging with drush @alias php:

>>> d()->root . ' should not be the same as ' . d('@hostmaster')->root
/var/aegir/platformpath should not be the same as /var/aegir/platformpath

I can't figure out why this is happening, but I figured we should change the way we detect hostmaster to something more reliable.