Lowercase the fork's image name in the PROD image example tests (#76143)

Image references must be lowercase. A fork whose owner has capital
letters made every Docker example build in this job fail with an
invalid reference format, so fork CI was red for those contributors
whatever their change. Breeze and the legacy image canary already
lowercase the repository name; this step did not.

Generated-by: Claude Opus 5

.github/workflows/additional-prod-image-tests.yml

@@ -197,7 +197,7 @@ jobs:
           DEFAULT_PYTHON_VERSION: "${{ inputs.default-python-version }}"
         run: "
           cd ./docker-tests && \
-          TEST_IMAGE=\"ghcr.io/$GITHUB_REPOSITORY/$DEFAULT_BRANCH\
+          TEST_IMAGE=\"ghcr.io/${GITHUB_REPOSITORY,,}/$DEFAULT_BRANCH\
           /prod/python$DEFAULT_PYTHON_VERSION\" \
           uv run pytest tests/docker_tests/test_examples_of_prod_image_building.py -n auto --color=yes"