Allow passing pex cache directory to speed up build

[mouadino]

When runing bazel build (or test) in standbox the pex cche (i.e. .pex/build)
ends up being created inside the sandbox directory which is deleted at the end
of bazel build (or test) which a second run will requires again to download
the same dependencies again and again.

[mouadino]

I know that saving cache for later usage may not be very hermetic since this later is not tracked by Bazel itself if you decide to set cache_dir to say $(@D) but still the speed improvement is dramatic (although the performance is better, it's still slow for our use case).