Call sudo /w -H in install_pip.sh as well

We updated other usage of sudo to pass -H when installing pip things,
to avoid creating a .cache directory in $STACK_USER's $HOME that is
owned by root. get-pip.py also ends up creating a ~/.cache, so we
need to update sudo usage there as well.

Closes-bug: #1405626
Related-bug: #1405732

Change-Id: If791b9b25d6a4280dab19117004184e57e78d038