cmd/go/internal/toolchain: apply the -modcacherw flag when downloading a module to...

cmd/go/internal/toolchain: apply the -modcacherw flag when downloading a module to determine what toolchain it needs

Fixes #64282.

Change-Id: I3f211c599ee70cb58254d0bc07eeb3c135124e58
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/555436
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>