D6976 introduced a subtle change that introduces two problems:

1. It attempts to run make-base-vm which requires elevated permissions (sudo).
2. The OSX_SDK gets wiped on every build since it no longer uses a shared location as a cache.

This patch partially restores that functionality, but no longer depends on an external
gitian-builder version. Teamcity will ensure the base image exists and an up-to-date
version of it is built using the source-controlled gitian-builder prior to running the
Gitian builds.

Run some tests in CI