Add a pip install step
We want to also install the wheels, not just generate them. To do that
we add a pip install -t <target> command. For this to work however, we
need to install into /opt/lib/python and amend the environment
accordingly to set PYTHONPATH and PATH in order for the libraries and
binaries to be accessible.
Reviewers: dduvall, thcipriani, Release-Engineering-Team
Differential Revision: https://phabricator.wikimedia.org/D1057