Fixes darwin-arm{,64} builds.
Child processes aren't allowed on iOS.

Change-Id: I9258ed4df757ec394ef6327dbda96f5b9705bcdd
Reviewed-on: https://go-review.googlesource.com/10142Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>