cmd/go: in TestScript, set $GOEXE instead of $exe

$GOEXE exists and is documented in 'go env', so $exe is redundant and
a bit confusing. Notably, mod_modinfo.txt already assumes that GOEXE
is set (even though it isn't), and thus fails on Windows.

After this CL, `go test cmd/go/...` passes on a windows-amd64-2016
builder. However, given that the $PATH on the builder is very minimal
(#32430) and network access is limited, tests that rely on binaries
(such as 'git') or external networking may still be broken.

Updates #25300

Change-Id: I9d80f2a0fbaa8bc35fa2205b6898aeccecda4e94
Reviewed-on: https://go-review.googlesource.com/c/go/+/181542


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>