Remove redundant cache dir normalization in _build_session
Remove redundant cache dir normalization in _build_session

Now the cache is normalized eagerly, this is not necessary here anymore.
assert the path is absolute to prevent regression.

sbidoul/pip,pfmoore/pip,pypa/pip,pypa/pip,pradyunsg/pip,pradyunsg/pip,pfmoore/pip,sbidoul/pip