always update the pip cache before installing packages

always update the pip cache before installing packages

We do this to get around a bug in the previous code that would never go
out to look for new packages if something in the cache satisfied the
requirements. Specifically, we wanted chacractl==0.0.5 installed, but
because 0.0.4 exists in the cache and satisified the requirement of
>=0.0.4 the cache was never updated to include 0.0.5.

Signed-off-by: Andrew Schoen <aschoen@redhat.com>