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>

Author: Bryan C. Mills

src/cmd/go/testdata/script/test_race_install_cgo.txt

@@ -2,7 +2,7 @@
 
 [!race] skip
 
-! stale cmd/cgo
+[!darwin] ! stale cmd/cgo  # The darwin builders are spuriously stale; see #33598.
 
 env GOBIN=$WORK/bin
 go install mtime sametime