Flaky post-PR failure

[3](https://github.com/RoboStack/ros-rolling/actions/runs/33316099505/job/99275421825#step:3:7804)
+ pixi run upload /home/runner/conda-bld//linux-aarch64/ros2-autoware-common-msgs-1.12.0-np2py314hbb80cda_25.conda --skip-existing
✨ Pixi task (upload): rattler-build upload prefix -c robostack-rolling --generate-attestation /home/runner/conda-bld//linux-aarch64/ros2-autoware-common-msgs-1.12.0-np2py314hbb80cda_25.conda --skip-existing
Error:   × Failed to generate attestation for ros2-autoware-common-msgs-1.12.0-
  │ np2py314hbb80cda_25.conda: Failed to sign attestation with Sigstore:
  │ Signing error: Failed to get timestamp: HTTP error: TSA returned status
  │ 400 Bad Request
  │ Upload aborted because attestation generation was requested but failed.
  │ 
  │ Troubleshooting:
  │ 1. Check that you're running in a supported CI environment (GitHub
  │ Actions, GitLab CI, etc.)
  │ 2. For GitHub Actions, ensure you have 'id-token: write' permission
  │ 3. Verify OIDC token is available and valid

For example:

• https://github.com/RoboStack/ros-rolling/actions/runs/33316099505/job/99275421825
• https://github.com/RoboStack/ros-rolling/actions/runs/33316058402/job/99285261758

Start with the linked GitHub Actions job logs and inspect the upload step that runs pixi run upload with attestation generation. Compare both example runs and verify the GitHub Actions OIDC and Sigstore timestamping context; done means the post-PR upload completes without the TSA 400 error.