snap shouldn't install bitmask-root
if I'm not wrong, in the case of the snap, install hooks are only executed first time the snap is installed.
in any case, it reduces the complexity if we can just install the polkit file system-wide (this should also be confined whenever it's possible to do that with snapcraft - as soon as some plugin for polkit is implemented), and then we point from the polkit file to a well-known path in the snap folder.
it probably will get messy when we also have a snap for bitmask, so we should have some way of detecting which app we're running.