Bugfix: Fix extended attributes handling for VSS snapshots

Restic was failing to backup extended attributes for VSS snapshots
after the fix for https://github.com/restic/restic/pull/4980.
Restic now correctly handles extended attributes for VSS snapshots.

https://github.com/restic/restic/pull/4998
https://github.com/restic/restic/pull/4980