Ensure Box.sandbox() doesn't leave lingering tmp-* directories

[gnidan]

Right now other Truffle projects (mostly truffle-core for tests) use this static method to create a Truffle project in a local temp directory.

These directories persist beyond the process lifecycle, so the local directory ends up getting quite polluted.

It'd be nice for these directories to automatically get cleaned up when the process exits. We almost certainly want to delete these temp directories for exit 0; non-zero exit codes may warrant leaving the directory around (in case we want to inspect what went wrong).