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
diff --git a/tools/install_pip.sh b/tools/install_pip.sh
index f30ad2b..d57a687 100755
--- a/tools/install_pip.sh
+++ b/tools/install_pip.sh
@@ -46,7 +46,7 @@
         curl -o $LOCAL_PIP $PIP_GET_PIP_URL || \
             die $LINENO "Download of get-pip.py failed"
     fi
-    sudo -E python $LOCAL_PIP
+    sudo -H -E python $LOCAL_PIP
 }