Thanks to Alin Gabriel Serdean for helping figure out the correct
solution here.
This adds --diable-pip-version-check because upgrading pip (as
suggested by the error that this option suppresses) causes pip
to fail, which is in turn a bug in pip that manifests only on
Windows: https://github.com/pypa/pip/issues/3383.
Signed-off-by: Ben Pfaff <blp@ovn.org> Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>