Use check-submodules action (#272)

Author: graebm

.github/workflows/ci.yml

@@ -118,3 +118,16 @@ jobs:
           python3 -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz', 'builder')"
           chmod a+x builder
           ./builder build -p ${{ env.PACKAGE_NAME }} --spec=downstream
+
+  check-submodules:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout Source
+      uses: actions/checkout@v2
+      with:
+        submodules: true
+        fetch-depth: 0
+    - name: Check Submodules
+      # note: using "@main" because "@${{env.BUILDER_VERSION}}" doesn't work
+      # https://github.com/actions/runner/issues/480
+      uses: awslabs/aws-crt-builder/.github/actions/check-submodules@main