Grant the user read access to external Persistent Storage

Since Tails 4.13 when a user has disabled persistence, or plugs another Tails with a TailsData partition, and tries to mount TailsData through Nautilus, they are met with an error: 'This location could not be displayed. You do not have the permissions necessary to view the contents of "TailsData"'. This is due to #7465 (closed) (!210 (merged)) where we indeed made the TailsData root world-non-readable, and did not consider this use case.

Note that this change indeed changes the permission, which might seem dangerous, but if the Tails is later booted it will self-heal, so all is ok.

Closes: #18050 (closed)