Use tmp location for tool cache to avoid issues deleting machines

When starting an LXC container for the local provider, if we put the tools tarballs in a bind mounted dir which lives under /var/lib/juju, the contents of this dir are deleted when the agent exits on machine deletion. Thus subsequent containers can't find tools. So we use a tmp dir instead.

Fixes: https://bugs.launchpad.net/juju-core/1.20/+bug/1379802