Use checkout action in nightly workflow (#5591)

Author: theorashid

.github/workflows/nightly.yml

@@ -32,6 +32,7 @@ jobs:
     needs: build-and-publish-nightly
     runs-on: ubuntu-latest
     steps:
+    - uses: actions/checkout@v2
     - name: Set up Python
       uses: actions/setup-python@v2
       with: