Adding checkout action because deploy pages requires it.

Author: germa89

.github/workflows/ci-build.yml

@@ -52,6 +52,8 @@ jobs:
     needs: docs_build
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v2
+
       - name: Set up Python
         uses: actions/setup-python@v1
         with: