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

When running `restic backup` with `--use-fs-snapshot` flag 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 requestor 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(the backup itself succeeds). Prevent this from occurring.


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