jenkins-build: make `pip install` cache all downloaded files

This reduces bandwidth use, reduces the secure risk since the tarball is
only downloaded once, and reduces the ability of a network observer to
profile what kind of build activity is happening on a machine.
include jenkins-build in source tarball
switch default 'Python Imaging' to Pillow, that's what is shipped

PIL is defunct, more or less, and only available via a non-pypi, insecure
download location with no way to verify the integrity.