add support for disabling relay via environment variable

client/internal/engine.go

@@ -942,7 +942,9 @@ func (e *Engine) handleRelayUpdate(update *mgmProto.RelayConfig) error {
 			return fmt.Errorf("update relay token: %w", err)
 		}
 
-		e.relayManager.UpdateServerURLs(update.Urls)
+		if !relayClient.IsDisableRelay() {
+			e.relayManager.UpdateServerURLs(update.Urls)
+		}
 
 		// Just in case the agent started with an MGM server where the relay was disabled but was later enabled.
 		// We can ignore all errors because the guard will manage the reconnection retries.