Cache the arduino prefs lookup

etc/kaleidoscope-builder.conf

@@ -70,7 +70,9 @@ get_arduino_pref() {
 dump_arduino_prefs() {
 # SKETCH and -build-path in this command are here because of a bug introduced in Arduino 1.8.10
 # https://github.com/arduino/arduino-builder/issues/341
-    prefs=$("${ARDUINO_BUILDER}" \
+    if [ "x${_ARDUINO_PREFS}x" == "xx" ]; then
+
+	    _ARDUINO_PREFS=$("${ARDUINO_BUILDER}" \
 		-hardware "${ARDUINO_PATH}/hardware" \
 		-hardware "${BOARD_HARDWARE_PATH}" \
 		${ARDUINO_TOOLS_FLAG:+"${ARDUINO_TOOLS_FLAG}"} ${ARDUINO_TOOLS_PARAM:+"${ARDUINO_TOOLS_PARAM}"} \
@@ -78,7 +80,8 @@ dump_arduino_prefs() {
 		-fqbn "${FQBN}" \
 	 	-build-path "${ARDUINO_PATH}" \
    		-dump-prefs  "${SKETCH_DIR}/${SKETCH}.ino" )
-    echo "$prefs"
+    fi
+    echo "$_ARDUINO_PREFS"
 
 }