Fix arduino_cron

Apparently since #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.

Author: jepler

.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