Updated API Diff Files are not being published

The updated API diff file is not being published as an artifact in the SDK diff test pipeline runs:

<img width="3156" height="635" alt="Image" src="https://github.com/user-attachments/assets/7448bda6-b4c3-457e-861c-5f6f8d2fc702" />

This makes fixing https://github.com/dotnet/source-build/issues/5378 difficult.