misc/ios: add support for device ids to the exec wrapper
misc/ios: add support for device ids to the exec wrapper

If set, GOIOS_DEVICE_ID specifies the device id for the iOS exec
wrapper. With that, a single builder can host multiple iOS devices.

Change-Id: If3cc049552f5edbd7344befda7b8d7f73b4236e2
Reviewed-on: https://go-review.googlesource.com/57296
Run-TryBot: Elias Naur <993af7099cb24ce1a88d64108ae1e8b0385f2fc0@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: JBD <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

Cofyc/go,golang/go,polyverse-security/go,josharian/go,cloudflare/go,cloudflare/go,nawawi/go,polyverse-security/go,polyverse-security/go,Cofyc/go,akutz/go,golang/go,akutz/go,polyverse-security/go,golang/go,CAFxX/go,Cofyc/go,cloudflare/go,Cofyc/go,polyverse-security/go,CAFxX/go,nawawi/go,golang/go,josharian/go,nawawi/go,akutz/go,cloudflare/go,codestation/go,codestation/go,akutz/go,josharian/go,codestation/go,akutz/go,cloudflare/go,cloudflare/go,christopher-henderson/Go,CAFxX/go,polyverse-security/go,josharian/go,CAFxX/go,Cofyc/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,polyverse-security/go,codestation/go,akutz/go,codestation/go,josharian/go,josharian/go,codestation/go,josharian/go,golang/go,CAFxX/go,Cofyc/go,golang/go,akutz/go,polyverse-security/go,Cofyc/go,golang/go,codestation/go,codestation/go,nawawi/go,nawawi/go,josharian/go,Cofyc/go,golang/go