Only run the GitHub Actions workflow on the Adafruit repository.

.github/workflows/workflow.yml

@@ -8,8 +8,23 @@ on:
     - cron: 0 10 * * *
 
 jobs:
+  check-repo-owner:
+    # This job is so the entire workflow will end successfully and give some
+    # output to explain why it hasn't run on a non-Adafruit fork.
+    runs-on: ubuntu-latest
+    steps:
+      - name: repository
+        env:
+          OWNER_IS_ADAFRUIT: ${{ startswith(github.repository, 'adafruit') }}
+        run: |
+          echo "This workflow will only run if Adafruit is the repository owner."
+          echo "Repository owner is Adafruit: $OWNER_IS_ADAFRUIT"
   build:
     runs-on: ubuntu-latest
+    # Only run the build on Adafruit's repository. Forks won't have the secrets.
+    # Its necessary to do this here, since 'schedule' events cannot (currently)
+    # be limited (they run on all forks' default branches).
+    if: startswith(github.repository, 'adafruit')
     steps:
     - uses: actions/checkout@v1
       with: