cmd/go: include internal/godebugs in TestNewReleaseRebuildsStalePackagesInGOPATH

cmd/go: include internal/godebugs in TestNewReleaseRebuildsStalePackagesInGOPATH

This fixes a lingering failure on the 'longtest' builders,
introduced in CL 476280.

For #57001.

Change-Id: I18872b1bc990dd16c0b86ebc5878c0a04ce9b2d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/485897
Run-TryBot: Bryan Mills <bcmills@google.com>
Commit-Queue: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>