cmd/go: update go_windows_test to use test go binary

Most of the cmd/go tests build the cmd/go binary and run that binary to
test it, but TestAbsolutePath used the GOROOT's cmd/go instead, which
makes debugging confusing and means that make.bash has to be run in each
iteration cycle. Update TestAbsolutePath to use the same go binary as
the rest of the cmd/go tests.

Change-Id: Ib4e8ae707b66f1f75ceb346b98358f5604fd28c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/256979


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>