Display a prompt message at the beginning of the build to prevent the…

… user from mistakenly thinking that the program is stuck when the arduino-cli starts slowly in some cases

src/upload/arduino.js

@@ -102,6 +102,7 @@ class Arduino {
             });
 
             const arduinoBuilder = spawn(this._arduinoCliPath, args);
+            this._sendstd(`Start building...\n`);
 
             arduinoBuilder.stderr.on('data', buf => {
                 const data = buf.toString();