csi: prevent in-use plugin GC from blocking volume GC (hashicorp#9141)

During CSI plugin GC, we don't return an error if the volume is in use,
because this is not an error condition. If we were to return an error during a
`nomad system gc`, we would not continue on to GC volumes.

But check for the specific error message fails if the GC is performed on a
worker rather than on the leader, due to RPC forwarding wrapping the error
message. Use a less specific test so that we don't return an error.

nomad/core_sched.go

@@ -860,7 +860,7 @@ func (c *CoreScheduler) csiPluginGC(eval *structs.Evaluation) error {
 			}}
 		err := c.srv.RPC("CSIPlugin.Delete", req, &structs.CSIPluginDeleteResponse{})
 		if err != nil {
-			if err.Error() == "plugin in use" {
+			if strings.Contains(err.Error(), "plugin in use") {
 				continue
 			}
 			c.logger.Error("failed to GC plugin", "plugin_id", plugin.ID, "error", err)