Add access token to C++ build test

This is necessary for downstream adopters.

.github/workflows/ccpp.yml

@@ -8,6 +8,9 @@ jobs:
     container: px4io/px4-dev-base-bionic:2019-10-24
     steps:
     - uses: actions/checkout@v1
+      with:
+        token: ${{ secrets.ACCESS_TOKEN }}
+        submodules: true
     - name: make
       run: make