Bugfix: Skip EA processing in Windows for volumes that do not support EA

Restic was failing to backup files on some windows paths like network
drives because of errors while fetching extended attributes.
Either they return error codes like windows.E_NOT_SET or
windows.ERROR_INVALID_FUNCTION or it results in slower backups.
Restic now completely skips the attempt to fetch extended attributes
for such volumes where it is not supported.

https://github.com/restic/restic/pull/4980
https://github.com/restic/restic/issues/4955
https://github.com/restic/restic/issues/4950