cmd/go: implement 'toolchain local'

cmd/go: implement 'toolchain local'

The actual selection code already worked
(except for the x/mod parser not reading the file),
so all that is necessary is a test.
For the test, move the version check up before
the module line presence check.

For #57001.

Change-Id: Iaa4f9b92d38fcfd99dc1665ec8d3eb0e52007bb4
Reviewed-on: https://go-review.googlesource.com/c/go/+/497555
TryBot-Bypass: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>