bug fdbbackup list: using list on a s3 blob store url generates incorrect url

When using an s3 blob store url with only access key and secret key, ( no session token ) the listed backups are incorrect.

Here's a sample output
```
user@host ~ % fdbbackup list -b 'blobstore://<accesskey>:<secretkey>@s3.us-east-1.amazonaws.com?bucket=<bucketname>&sc=0'
blobstore://<accesskey>:<secretkey>:<secretkey>@s3.us-east-1.amazonaws.com/<backupname>?bucket=<bucketname>&sc=0
```