[conftest] Fix EROFS fallback for kernel downloads via HfApi (#47791)

[conftest] fix EROFS fallback for kernel downloads via HfApi

The CI read-only cache fallback wrapped `huggingface_hub.hf_hub_download`
but missed `huggingface_hub.hf_api.hf_hub_download` — the module-level
name that `HfApi.hf_hub_download` resolves at call time. Third-party
libraries such as `kernels` call `api.hf_hub_download(...)` (HfApi
instance), so their downloads bypassed the EROFS retry entirely.

Patch that reference explicitly; `snapshot_download` is unaffected
because it imports `hf_hub_download` from `file_download.py` directly.

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>