cmd/go: add Context parameter to download function

cmd/go: add Context parameter to download function

Updates #38714

Change-Id: Ie5c7761ec003f84e649fa4c90be184a5ff6a0879
Reviewed-on: https://go-review.googlesource.com/c/go/+/419554
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Joedian Reid <joedian@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>