cmd/api: skip tests when 'os/exec' is supported but 'go build' is not

cmd/api: skip tests when 'os/exec' is supported but 'go build' is not

This may fix the android-.*-emu builders, which are currently broken.

For #47257.

Change-Id: I370dad2cb8031f8f5fdfbeb9c284c4f79f58d929
Reviewed-on: https://go-review.googlesource.com/c/go/+/446638
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>