cmd/go: fix GOTOOLCHAIN parsing for +auto

cmd/go: fix GOTOOLCHAIN parsing for +auto

The call from toolchain.Select to gover.FromToolchain was passing the
wrong argument but this was masked by gover.IsValid being a little bit
too lax.

Fix and test IsValid, which then breaks the existing gotoolchain_local
test, and then fix toolchain.Select to fix the test.

Fixes #61068.

Change-Id: I505ceb227457d6a51bd5e47f009b2fb1010c0d1f
Reviewed-on: https://go-review.googlesource.com/c/go/+/508820
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>