Avoid error due to attempting to run the virtual display command on systems that don't have the necessary program

This command is only required for headless operation with older versions of the Arduino IDE. In most cases where sbin/start-stop-daemon doesn't exist this command is not necessary anyway.

Author: per1234

arduino-ci-script.sh

@@ -1407,6 +1407,8 @@ echo "Build Timestamp (UTC)"$'\t'"Build"$'\t'"Job"$'\t'"Job URL"$'\t'"Build Trig
 
 # Start the virtual display required by the Arduino IDE CLI: https://github.com/arduino/Arduino/blob/master/build/shared/manpage.adoc#bugs
 # based on https://learn.adafruit.com/continuous-integration-arduino-and-you/testing-your-project
-/sbin/start-stop-daemon --start $ARDUINO_CI_SCRIPT_QUIET_OPTION $ARDUINO_CI_SCRIPT_VERBOSITY_OPTION --pidfile /tmp/custom_xvfb_1.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :1 -ac -screen 0 1280x1024x16
-sleep 3
-export DISPLAY=:1.0
+if [ -e /usr/bin/Xvfb ]; then
+  /sbin/start-stop-daemon --start $ARDUINO_CI_SCRIPT_QUIET_OPTION $ARDUINO_CI_SCRIPT_VERBOSITY_OPTION --pidfile /tmp/custom_xvfb_1.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :1 -ac -screen 0 1280x1024x16
+  sleep 3
+  export DISPLAY=:1.0
+fi