Fix CI workflow.

.github/workflows/on-pr.yaml

@@ -26,6 +26,7 @@ jobs:
     name: Compare conscise migration schema with incremental migration.
     runs-on: ubuntu-latest
     steps:
+      - uses: actions/checkout@v2
       - name: Run database
         run: docker-compose -f docker-compose.test.yml up -d test-db statediff-migrations
       - name: Test incremental migration