fix: MQTT connection on Portduino/Linux native nodes (#10330)

isConnectedToNetwork() always returned false on ARCH_PORTDUINO
because none of HAS_WIFI, HAS_ETHERNET, or USE_WS5500 are defined
for Linux native builds. This caused wantsLink() to always return
false, preventing the MQTT thread from ever connecting at boot.

Fix: return true for ARCH_PORTDUINO since Linux always has network
access available.

Co-authored-by: Jonathan Bennett <jbennett@incomsystems.biz>

src/mqtt/MQTT.cpp

@@ -350,6 +350,8 @@ inline bool isConnectedToNetwork()
     return WiFi.isConnected();
 #elif HAS_ETHERNET
     return Ethernet.linkStatus() == LinkON;
+#elif defined(ARCH_PORTDUINO)
+    return true;
 #else
     return false;
 #endif