cmd/go: preserve LIBRARY_PATH and C_INCLUDE_PATH for script tests

cmd/go: preserve LIBRARY_PATH and C_INCLUDE_PATH  for script tests

In bespoke build environments default libraries may be specificied with
LIBRARY_PATH, C_INCLUDE_PATH enviroment variables to overide the system
(i.e glibc). Allow them though to allow cgo testing of these enviroments.

Fixes #50985

Change-Id: I7497a7715d9b635a6ae97efaab94a7ff01cdf8e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/383334
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Trust: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>