Modify qvm-sdk make target to (optionally) use a unique cache.

[notmgsk]

Currently, make qvm-sdk will kill ~/.cache/common-lisp/<sbcl-cache>. Ideally, it would work with its own cache so that (1) it pulls fresh packages when building, and (2) doesn't affect the current cache, which might be unexpected behaviour.

[appleby]

From the sdk build's point of view, I think this is usually run inside a fresh docker container via the docker-sdk target. From the standpoint of testing things out without getting docker involved, it should be safe to blow the cache away. In theory it should just mean a potentially slower build the next time you build qvm.