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

On Windows, when creating a backup with the `--use-fs-snapshot` option, restic
read extended metadata from the original filesystem path instead of from the
snapshot. This could result in errors if files were removed during the backup
process.

This issue has now been resolved.

https://github.com/restic/restic/issues/5063
https://github.com/restic/restic/pull/5097
https://github.com/restic/restic/pull/5099