PMSA0031 sensors require ~3secs before coming up on I2C

As reported by @MALAONE1 and debugged by @shodan8192 , PMSA0031s
on a RAK4631 take 3 seconds before they can become detectable on I2c.

Add a delay(4000) before I2C scan if the air quality sensor pin
is defined.

Fixes #3690

src/main.cpp

@@ -393,6 +393,7 @@ void setup()
     // RAK-12039 set pin for Air quality sensor
     pinMode(AQ_SET_PIN, OUTPUT);
     digitalWrite(AQ_SET_PIN, HIGH);
+    delay(4000); // Detectable on I2C after ~3 seconds.
 #endif
 
 #ifdef T_DECK