cmd/go: rewrite TestScript/cgo_stale_precompiled to be agnostic to staleness

The configuration set by x/build/cmd/releasebot causes runtime/cgo to
be stale in the darwin/amd64 release (see #36025, #35459).
That staleness is mostly benign because we can reasonably assume that
users on macOS will either disable CGO entirely or have a C compiler
installed to rebuild (and cache) the stale packages if needed.

Fixes #50892
Fixes #50893
Updates #46347

Change-Id: Ib9ce6b5014de436264238f680f7ca4ae02c9a220
Reviewed-on: https://go-review.googlesource.com/c/go/+/381854
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

Author: Bryan C. Mills

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

@@ -4,14 +4,25 @@
 
 [!cgo] skip
 
-# Control case: net must not already be stale.
-! stale net
+# This test may start with the runtime/cgo package already stale.
+# Explicitly rebuild it to ensure that it is cached.
+# (See https://go.dev/issue/50892.)
+#
+# If running in non-short mode, explicitly vary CGO_CFLAGS
+# as a control case (to ensure that our regexps do catch rebuilds).
+
+[!short] env GOCACHE=$WORK/cache
+[!short] env CGO_CFLAGS=-DTestScript_cgo_stale_precompiled=true
+go build -x runtime/cgo
+[!short] stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
 
 # https://go.dev/issue/47215: a missing $(go env CC) caused the precompiled net to be stale.
 [!plan9] env PATH=''  # Guaranteed not to include $(go env CC)!
 [plan9] env path=''
-! stale net  # issue #47215
+go build -x runtime/cgo
+! stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'
 
 # https://go.dev/issue/50183: a mismatched GOROOT_FINAL caused net to be stale.
 env GOROOT_FINAL=$WORK${/}goroot
-! stale net
+go build -x runtime/cgo
+! stderr '[/\\]cgo'$GOEXE'["]? .* -importpath runtime/cgo'