Pin older checkout for container compat

Container does not have new enough git for checkout v2

.github/workflows/CI.yml

@@ -138,7 +138,7 @@ jobs:
       # github actions requires root to access mounted volumes
       options: --user root
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v1
         with:
           submodules: 'recursive'
       - run: |