Use pip's http-v2 cache

[woodruffw]

With pypa/pip#11143, pip uses $CACHE/http-v2 instead of $CACHE/http. This new directory uses a separated body format, rather than collecting the cached responses into a single serialized file each (both header and body).

As a result, pip-audit is currently only pulling from the older http cache, which will slowly go away in user installs over time. We should switch _cache.py to mimic and reuse the new http-v2 cache.

More context: here is the current SafeFileCache implementation within pip: https://github.com/pypa/pip/blob/34e70ba5964824ac8c52773c76c658923ee36194/src/pip/_internal/network/cache.py#L32