Sometimes ideviceinstaller fails to install the app. Retry a few
times before giving up.

For the iOS builder.

Change-Id: Ib066ffd4f97ae8d22c0fa9a78ea4d04f67c17410
Reviewed-on: https://go-review.googlesource.com/111055
Run-TryBot: Elias Naur <elias.naur@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>