This is unnecessary because Darwin builds already cannot create
`set{u,g}id` files due to the minimal sandbox policy, because we can
forbid messing with the top‐level build directory directly in the
sandbox rules, and because Darwin builds can trivially avoid isolation
through temporary directories right now anyway.
This does regress the build directory isolation for builds with
`sandbox = false`, but I can’t imagine that mattering given the
above. The sandbox change prepares us for a world where we close
off shared temporary directories for `sandbox = true` builds and try
harder to achieve proper isolation on Darwin, but probably doesn’t
have a meaningful security impact one way or another for now.
With this change, we get down to 41 byte build directory paths on
Darwin, a ~2–3½ byte improvement over the old status quo. We can
also restore the 0710 permissions on Linux.
Change-Id: I6a6a6964a681c0365241fe7234831db656b76799
