cmd/go: fix rebuild after installation of new Go release

The loading of zversion.go was expecting it to be in
package runtime, but it moved to runtime/internal/sys.
Worse, the load was not checking the error.

Update the path, check the error, add a test.

Fixes #14176.

Change-Id: I203c40afe1448875581415d5e42c29f09b14545d
Reviewed-on: https://go-review.googlesource.com/19180


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>