cmd/go: use gover.Local for $goversion in TestScript

cmd/go: use gover.Local for $goversion in TestScript

Previously we used the highest Go build tag found in the build
configuration, which matches gover.Local for development toolchains
(it is always a bare language version), but is too low for releases.

Updates #57001.

Change-Id: I74c2f7ab06231858eee99ecd11ed3759853e01ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/503537
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>