cmd/go: fix windows test failure for test_buildinfo_godebug_issue68053

cmd/go: fix windows test failure for test_buildinfo_godebug_issue68053

Fix required adding $GOEXE because windows will produce a .exe file.

Fixes: #68673
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: I25206de362ee4be6a9c54bd1baa405f325d79313
Reviewed-on: https://go-review.googlesource.com/c/go/+/602095
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>