Don't reuse pip builddir when mirroring.

* jeepyb/cmd/run_mirror.py: Instead of reusing the download cache
directory as the build directory, let it create a fresh temporary
builddir for each pip install command. This will significantly
increase run times, but if pip install finds an unpacked package
already in its builddir which meets the requirements specified it
will get reused even if a newer version is available on pypi.

Change-Id: I5f7f988315bef5985cc18ed8ba0f18ea2f4b2921
Reviewed-on: https://review.openstack.org/23971
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins