cmd/go: pass GoVersion in vet config

cmd/go: pass GoVersion in vet config

When invoking a vet tool with -vettool (or vet itself),
we need to pass the package's GoVersion to use when
analyzing the package.

The test of this behavior is in the x/tools/go/analysis CL 507880.

For #61176.

Change-Id: I3b5a90fcd19a0adc7fb29366e106e18f722fc061
Reviewed-on: https://go-review.googlesource.com/c/go/+/507976
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>