Update ci.yml

Author: SilanHe

.github/workflows/ci.yml

@@ -30,6 +30,7 @@ jobs:
       with:
         token: ${{ steps.generate-token.outputs.token }}
         repository: aws/aws-durable-execution-sdk-python
+        ref: 'main'
         # Make sure the value of GITHUB_TOKEN will not be persisted in repo's config
         persist-credentials: false