cmd/go: switch to newer toolchain in go get as needed

cmd/go: switch to newer toolchain in go get as needed

If we run 'go get go@1.40' or 'go get m@v' where m has a go.mod
that says 'go 1.40', we need to write a new go.mod that says 'go 1.40'.
But we can't be sure we know how to write a Go 1.40-compatible go.mod.
Instead, download the latest point release of Go 1.40 and invoke it to
finish the get command.

For #57001.

Change-Id: I4133fc3c2ecf91226a6c09a3086275ecc517e223
Reviewed-on: https://go-review.googlesource.com/c/go/+/498118
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>