On (at least) macOS 10.12, the `security cms` subcommand used by the
iOS detection script will output an error to stderr. The command
otherwise succeeds, but the extra line confuses a later parsing step.

To fix it, use only stdout and ignore stderr from every command run
by detect.go.

For the new iOS builders.

Change-Id: Iee426da7926d7f987ba1be061fa92ebb853ef53d
Reviewed-on: https://go-review.googlesource.com/36059Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>