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>