cmd/go: ensure git attributes are set

This change disables the export-subst and export-ignore attributes when
creating zip files for modules. This is done to prevent the ziphash for
a given repo/revision from differing based on variables such as git
version or size of repo. The full rational for this change is detailed
here:

    https://github.com/golang/go/issues/27153#issuecomment-420763082

Fixes #27153

Change-Id: Ib33f525d91d2581fa0b5d26e70d29620c7e685e9
Reviewed-on: https://go-review.googlesource.com/c/135175


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