Fix arduino_cron

Apparently since adafruit#2497, in which the extra 'checkout' line was lost,
we have not actually been building the arduino artifacts during CI.

The line may have looked like a mistake or duplicate when the CI was
revised.

.github/workflows/arduino_cron.yml

@@ -68,6 +68,11 @@ jobs:
  - uses: actions/setup-python@v5
  with:
  python-version: "3.x"
+
+ # Checkout the learn repo itself
+ - uses: actions/checkout@v4
+
+ # Checkout the CI scripts
  - uses: actions/checkout@v4
  with:
  repository: adafruit/ci-arduino