misc/cgo/testshared: do not write to GOROOT

misc/cgo/testshared: do not write to GOROOT

Instead of installing shared libraries to GOROOT/pkg, clone the
necessary files into a new GOROOT and run there.

Given that we now have a build cache, ideally we should not need to
install into GOROOT/pkg at all, but we can't fix that during the 1.14
code freeze.

Updates #28387
Updates #28553
Updates #30316

Change-Id: I83084a8ca29a5dffcd586c7fccc3f172cac57cc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/208482
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>