add missing checkout action

Author: aajtodd

.github/workflows/codebuild-ci.yml

@@ -26,6 +26,8 @@ jobs:
   service-check-batch:
     runs-on: ubuntu-latest
     steps:
+      - name: Checkout sources
+        uses: actions/checkout@v2
       - name: Configure AWS Credentials
         uses: aws-actions/configure-aws-credentials@v1
         with: