automatic tox env cachedir has no ux indication and triggers always

[RonnyPfannschmidt]

• Default to $TOX_ENV_DIR/pytest_cache #4270 introduced a potentially confusing/problematic behaviour of automnatically using a pytest cache in a tox env dir

that caused #8818 which was fixxed by a manual override

pytest should indicate when it picks different cache dirs and we should consider how to deal with tox is local vs tox is jsut used to invoke