cmd/go: fix TestScript/gotoolchain* when go.env doesn't set GOTOOLCHAIN=auto

cmd/go: fix TestScript/gotoolchain* when go.env doesn't set GOTOOLCHAIN=auto

Some of the TestScript/gotoolchain* tests assume that go.env contains
GOTOOLCHAIN=auto, but that's not always the case, for example CI
environments may set it to `local` to avoid downloading a new toolchain.

This commit fixes the tests to work regardless of the value of
GOTOOLCHAIN in go.env.

Fixes #60685

Change-Id: Ieda22574f8a028893762274cf9db721c9d69bf7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/502035
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Eli Bendersky <eliben@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>