ci: remove HF_HOME=/mnt/cache — runners don't have that volume

All tests failed with PermissionError at /mnt/cache. Unlike transformers'
Docker-based CI which mounts a persistent cache volume, our runners run
bare-metal without /mnt/cache. Removing HF_HOME falls back to the default
~/.cache/huggingface which is always writable. Models download fresh each
nightly run — acceptable for a smoke test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Titus-von-Koeller

.github/workflows/tests-integration-nightly.yml

@@ -28,7 +28,6 @@ env:
   PYPI_INDEX: "https://download.pytorch.org/whl/cu128"
   BNB_WHEEL_URL: "https://github.com/bitsandbytes-foundation/bitsandbytes/releases/download/continuous-release_main/bitsandbytes-1.33.7.preview-py3-none-manylinux_2_24_x86_64.whl"
   CUDA_VISIBLE_DEVICES: "0,1"
-  HF_HOME: /mnt/cache
 
 jobs:
   # ─── Downstream test jobs ─────────────────────────────────────────────────