D6976 introduces three distict issues in gitian.sh:

1. It attempts to run make-base-vm which requires elevated permissions (sudo) to work correctly.
2. The OSX_SDK gets wiped on every build since it no longer uses a shared location as a cache.
3. The gitian-builder/var/lxc.config is now missing.

While working on a fix for the first (D6985), the second became apparent, so I tacked it on.
But upon testing in CI, the third became apparent and the right course of action is less clear.

It's best to revert this change to unbreak the Gitian builds until we have a solid plan of
action for the above items.

CI