Set preview docs in PRs to overwrite (#7116)

This sets the upload to S3 for docs previews in PRs to overwrite. This
is generally in line with standard S3 behavior and should prevent a CI
error where repeated builds in PRs fail to upload after the first run,
e.g.
[here](https://github.com/pytorch/vision/actions/runs/17506203012?pr=9207).

Author: zklaus

.github/workflows/linux_job_v2.yml

@@ -350,6 +350,8 @@ jobs:
           path: ${{ env.RUNNER_DOCS_DIR }}
           # ${{ env.repository }} is $OWNER/$REPO
           s3-prefix: ${{ env.REPOSITORY }}/${{ github.event.pull_request.number }}
+          # Overwrite doc previews for the same PR
+          overwrite: true
 
       - name: Teardown Linux
         if: always() && steps.check_container_runner.outputs.IN_CONTAINER_RUNNER == 'false'