cmd/go: restore go.mod files during toolchain selection

cmd/go: restore go.mod files during toolchain selection

They have to be renamed to _go.mod to make a valid module.
Copy them back to go.mod so that 'go test cmd' has a better
chance of working.

For #57001.

Change-Id: Ied6f0dd77928996ab322a55c5606d7f75431e362
Reviewed-on: https://go-review.googlesource.com/c/go/+/504118
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>