buildserver: vagrant-cachier conflicts with custom apt cache

buildserver: vagrant-cachier conflicts with custom apt cache

The technique where /var/cache/apt is mounted as a shared folder conflicts
with vagrant-cachier's workings.  Therefore, ignore vagrant-cachier if the
user selects ./makebuildserver's custom apt cache.  The shared folder way
has the advantage for CI builds of storing the cache outside of
VAGRANT_HOME, which is set to be in the git project.  That gets wiped by
`git clean -fdx` on each CI build.