workload: incorrect check for split at / scatter with cluster virtualization

**Describe the problem**

In `workload/workloadsql/workloadsql.go`:

```go
if strings.Contains(err.Error(), errorutil.UnsupportedUnderClusterVirtualizationMessage) {
// We don't care about split errors if we're running a workload
// with virtual clusters; we can't do them so we'll just continue.
break
}
```

1) SPLIT AT and SCATTER are possible with virtual clusters with the right capability
2) even when pointing `workload` to a secondary tenant without the capability, the error should be "capability missing", not "operation not supported". So the error check is incorrect.

Epic: CRDB-26687.

Jira issue: CRDB-30917