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 <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: JBD <jbd@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>