cmd/go: allow toolchain upgrades in 'go mod download' when we would already allow go.mod updates
This fixes an inconsistency that was introduced in CL 537480 and noted
in the review on CL 539697.
In particular, 'go mod download' already updates the go.mod file when
other kinds of updates are needed. (#45551 suggested that it should
not do so, but that part of the change was not implemented yet;
finishing that change is proposed as #64008.)
Updates #62054.
Change-Id: Ic659eb8538f4afdec0454737e982d42ef8957e56
Reviewed-on: https://go-review.googlesource.com/c/go/+/540779
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>