Add CI test to check builtin links weekly

#332

.github/workflows/verify_builtins_downloads.yml

@@ -0,0 +1,30 @@
+# Test that builtins download every week
+# Downloads ~1 GB of data and runs in a few minutes (Nov 5, 2021)
+
+name: Builtins download check
+
+# Controls when the workflow will run
+on:
+  schedule:
+    # https://crontab.guru/#5_1_1_*_*
+    # cant use special cron keys like @weekly in GitHub Actions
+    - cron:  '5 1 * * 1'
+
+  # Allows you to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+jobs:
+  builtins:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v2
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+      - name: Install Python dependencies
+        run: python -m pip install --upgrade tox
+      - name: Run tests
+        run: tox -e builtins