Expose scratch_dir in the Starlark analysis context

Starlark target kind implementations now receive ctx["scratch_dir"] alongside ctx["build_dir"]. build_dir remains the declared output namespace for targets, while scratch_dir is an analysis-chosen path for action-private helper files such as the Windows rustc response file, keeping those helpers from getting mixed with durable build outputs. The new key is documented in the public module reference, and target kind authors can rely on it from now on.