Update Rust nightly version
Update Rust nightly version

Would have jumped to 2019-09-18, but that seems to have issues on
Windows. I tried 2019-08-01 and 2019-09-01, and the August release
works. There may be a later one that also works, but finding that can
come later.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat