Switch flag definitions to be on pflag instead of flag in `package go/vt/mysqlctl/filebackupstorage`

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

```
$ git grep -E "\bflag\.[A-Z]" -- go/vt/mysqlctl/filebackupstorage/*.go
go/vt/mysqlctl/filebackupstorage/file.go:       FileBackupStorageRoot = flag.String("file_backup_storage_root", "", "root directory for the file backup storage")
```