fix(ci): Use absolute path for PYTHONPATH in upload docs workflow

The upload-adk-docs-to-vertex-ai-search workflow failed with ModuleNotFoundError: No module named 'adk_answering_agent' because the relative PYTHONPATH path did not resolve correctly under some CI environment execution states. Resolved by using the absolute ${{ github.workspace }} path.

Change-Id: I18c72b671f33c134bd08601b4a97671a1b1b0fa9