roachtest: use correct warehouse number in import

In 0a94874 we introduced a bug which
hard-coded that we'd always use 1 warehouse in tpcc import roachtests.
This is now fixed.

Release note: None

pkg/cmd/roachtest/tests/import.go

@@ -150,7 +150,7 @@ func registerImportTPCC(r registry.Registry) {
 			} else {
 				defer hc.Done()
 			}
-			cmd := fmt.Sprintf(workloadStr, 1)
+			cmd := fmt.Sprintf(workloadStr, warehouses)
 			// Tick once before starting the import, and once after to capture the
 			// total elapsed time. This is used by roachperf to compute and display
 			// the average MB/sec per node.