[release-branch.go1.21] cmd/go/internal/toolchain: apply the -modcacherw flag when downloading a module to determine what toolchain it needs
Fixes #64497.
Updates #64282.
Change-Id: I3f211c599ee70cb58254d0bc07eeb3c135124e58
Reviewed-on: https://go-review.googlesource.com/c/go/+/555436
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
(cherry picked from commit
cc38c68ae09fa591697a4239a7dedd2efe386995)
Reviewed-on: https://go-review.googlesource.com/c/go/+/559198
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>