cli/debug: clusters folder missing in MT cluster

A recent support ticket included a debug.zip from a cluster that contained multiple tenants with service mode shared, however the zip did not include a 'clusters' folder, or otherwise give any indication that there was a separate tenant virtual cluster. This was a little confusing, as it meant there were log files that mentioned specific job IDs or descriptor IDs that did not exist in the (only) jobs or descriptor tables in the zip.

Due to the issue that caused the support escalation in the first place, I believe the secondary tenant was failing to complete boot of its SQL server, so it would be very likely that it would be impossible to actually include query and dump its system tables, but I would typically expect a failure to query a system table expected to be included in a zip to produce the customary system.X.txt.err file signaling why it isn't there. In this case there was just no clusters folder at all, rather than one with a collection of .err files.

@dhartunian says:
> tenant discovery happens during the initial phase of debug zip so my guess is that if the tenant’s not responding it doesn’t even make it into the list of tenants to query over and hence is “invisible”.

Internal link: https://github.com/cockroachlabs/support/issues/3445

Jira issue: CRDB-55060