Fix ruff SIM108 lint error in conftest.py

[Copilot]

Pre-commit CI was failing due to a ruff SIM108 violation in tests/conftest.py.

Change

• Replaced if/else block with a ternary expression for cache_dir assignment:

# Before
if worker_id == "master":
    cache_dir = tmp_path / "cachier_cache"
else:
    cache_dir = tmp_path / f"cachier_cache_{worker_id}"

# After
cache_dir = tmp_path / "cachier_cache" if worker_id == "master" else tmp_path / f"cachier_cache_{worker_id}"

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