Bugfix: Prevent Windows VSS event log 8194 warnings for backup with fs snapshot

When running `backup` with the `--use-fs-snapshot` option in Windows with admin rights, event logs like

```
Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface.  hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requester process. 

Operation:
   Gathering Writer Data

Context:
   Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Name: System Writer
   Writer Instance ID: {54b151ac-d27d-4628-9cb0-2bc40959f50f}
```

are created several times even though the backup itself succeeds. This has now been fixed.

https://github.com/restic/restic/issues/5169
https://github.com/restic/restic/pull/5170
https://forum.restic.net/t/windows-shadow-copy-snapshot-vss-unexpected-provider-error/3674/2
