Bugfix: Correctly `backup` extended metadata when using VSS on Windows

On Windows, when creating a backup using the `--use-fs-snapshot` option,
then the extended metadata was not read from the filesystem snapshot. This
could result in errors when files have been removed in the meantime.

This issue has been resolved.

https://github.com/restic/restic/issues/5063
https://github.com/restic/restic/pull/5097