fix bug 1035519 - Stop destroying pip-cache

Keep the pip-cache around so that peep can do it's job by making sure
packages have not change out from under us. Also reduces build time, as
package no longer have to be re-downloaded.
Merge pull request #2170 from bramwelt/fix-bug-1035519-keep-pip-cache

fix bug 1035519 - Stop destroying pip-cache