cmd/go: skip the cmd/cgo staleness check on darwin builders

cmd/go: skip the cmd/cgo staleness check on darwin builders

Some of the darwin-amd64 builders are providing a stale environment.
Let's un-break them while we investigate.

Updates #37573
Updates #33598

Change-Id: I8b79778fe4d5aa916557c1ba89fa9c776d130b01
Reviewed-on: https://go-review.googlesource.com/c/go/+/223925
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>