misc/ios: ignore stderr from iOS tools
misc/ios: ignore stderr from iOS tools

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/36059
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>
Run-TryBot: Elias Naur <993af7099cb24ce1a88d64108ae1e8b0385f2fc0@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

Cofyc/go,CAFxX/go,Cofyc/go,cloudflare/go,golang/go,vsekhar/elastic-go,derekmarcotte/go,ImJasonH/go,nawawi/go,josharian/go,Cofyc/go,linux-on-ibm-z/go,polyverse-security/go,ImJasonH/go,derekmarcotte/go,codestation/go,codestation/go,linux-on-ibm-z/go,Cofyc/go,polyverse-security/go,polyverse-security/go,ImJasonH/go,golang/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,golang/go,akutz/go,CAFxX/go,polyverse-security/go,codestation/go,josharian/go,akutz/go,golang/go,CAFxX/go,derekmarcotte/go,akutz/go,linux-on-ibm-z/go,Cofyc/go,golang/go,cloudflare/go,vsekhar/elastic-go,Cofyc/go,codestation/go,josharian/go,cloudflare/go,vsekhar/elastic-go,ImJasonH/go,cloudflare/go,golang/go,codestation/go,vsekhar/elastic-go,cloudflare/go,codestation/go,nawawi/go,polyverse-security/go,akutz/go,josharian/go,ImJasonH/go,vsekhar/elastic-go,nawawi/go,nawawi/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,vsekhar/elastic-go,linux-on-ibm-z/go,josharian/go,derekmarcotte/go,nawawi/go,linux-on-ibm-z/go,nawawi/go,nawawi/go,vsekhar/elastic-go,CAFxX/go,ImJasonH/go,linux-on-ibm-z/go,josharian/go,derekmarcotte/go,cloudflare/go,polyverse-security/go,cloudflare/go,nawawi/go,derekmarcotte/go,CAFxX/go,Cofyc/go,CAFxX/go,josharian/go,golang/go,josharian/go,polyverse-security/go,CAFxX/go,ImJasonH/go,codestation/go,akutz/go,christopher-henderson/Go,ImJasonH/go,derekmarcotte/go,Cofyc/go,polyverse-security/go,vsekhar/elastic-go,linux-on-ibm-z/go,akutz/go,akutz/go