mqtt-all.py example: use retained publications

Per [the change I suggested to the Pico-based Enviro code](pimoroni/enviro#2), with this change, the MQTT messages will be published with the retain flag set, so that if a consumer is not subscribed, the most recent set of readings can still be read by a future subscriber later. 

This supports the [homebridge-enviroplus plugin](mhawkshaw/homebridge-enviroplus#2 (comment)) better, so that a publication is more likely to exist on the topic even if the Pi with Enviroplus / this example code is not running at the time. 

Additional update to consider would be to align the message data formats between this sample and the newer Enviro product line.

Author: andypiper

examples/mqtt-all.py

@@ -262,7 +262,7 @@ def main():
                 values.update(pms_values)
             values["serial"] = device_serial_number
             print(values)
-            mqtt_client.publish(args.topic, json.dumps(values))
+            mqtt_client.publish(args.topic, json.dumps(values), retain=True)
             display_status(disp, args.broker)
             time.sleep(args.interval)
         except Exception as e: