Let pip unpack packages in the build directory
Let pip unpack packages in the build directory

This change is meant to fix intermittent broken builds on Centos 7.

Depending on your system, if you leave out the `--build DIR` argument to
`pip install`, you may end up with pip trying to unpack packages in
`/tmp/something`. Then on the next build, things may break, because the
stuff in `/tmp` did not get cleaned up.

felixbuenemann/omnibus-gitlab,copystudy/omnibus-gitlab,felixbuenemann/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,gitlabhq/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,copystudy/omnibus-gitlab,aleksandrs-ledovskis/omnibus-gitlab,felixbuenemann/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,copystudy/omnibus-gitlab,gitlabhq/omnibus-gitlab