cmd/go: make module .zip files group/world readable

cmd/go: make module .zip files group/world readable

os.CreateTemp in downloadZip leaves downloaded .zip files readable only
by the owner. Make them group/world readable.

Fixes #52765

Change-Id: Iace13e4ad813201a533a1a5fc0c6d9b2e5349a42
Reviewed-on: https://go-review.googlesource.com/c/go/+/404854
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>