os/exec: skip unsupported test on Android

The underlying system call tested by TestCredentialNoSetGroups
is blocked on Android.

Discovered while running all.bash from an Android device; the syscall
is only blocked in an app context.

Change-Id: I16fd2e64636a0958b0ec86820723c0577b8f8f24
Reviewed-on: https://go-review.googlesource.com/c/go/+/170945


Run-TryBot: Elias Naur <mail@eliasnaur.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>