fix(codecov): update codecov-upload.yml with a git checkout step (#34929)

### Reason for this change

Fixing codecov report uploads

Previous attempts : #34845, #34814

### Description of changes

Checkout the github repo before doing an upload


- [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: kumvprat

.github/workflows/codecov-upload.yml

@@ -20,6 +20,8 @@ jobs:
       github.event.workflow_run.conclusion == 'success'
 
     steps:
+      - name: Checkout
+        uses: actions/checkout@v4
       - name: Download Artifacts
         uses: actions/download-artifact@v4
         with: