Test-freshness plugin crashes any pytest run with the cache provider disabled

[BrennanTM]

NaviNIBS/util/testing/freshness.py (resolveTestWorkingDir, ~line 80) uses config.cache unguarded. Because the plugin is registered as a pytest11 entry point, any pytest invocation in the environment with the cache provider disabled fails at collection for every test:

$ pytest -p no:cacheprovider tests/test_util
INTERNALERROR> AttributeError: 'Config' object has no attribute 'cache'

NAVINIBS_TEST_FRESHNESS=off doesn't guard this call site (the env-var check happens later).

Suggested fix: cache = getattr(config, 'cache', None), fall back to defaultTestWorkingDir() when it's absent, and honor the env-var opt-out before touching the cache at all.