Move pbuilder cache from /var/cache/ to cache/

/var disk space is at a premium, instead use our cache/ dir that's
more likely to sit on a file system with more space. Also makes the
build more self contained; remove the entire tree and no cruft left.

Change-Id: I65d6f5c570f9c830489f795feb38d5b9cd0444cc
1 file changed