cmd/go: set GOWORK=off explicitly in script tests that cd to GOROOT

cmd/go: set GOWORK=off explicitly in script tests that cd to GOROOT

Some folks working on the Go project may have a go.work file in
GOROOT/src in order to test changes in x repos. 'go test cmd/go'
should not fail if that is the case.

For #58767.

Change-Id: I0e57b15fb1d3e4abc4903c177434626c9f125cae
Reviewed-on: https://go-review.googlesource.com/c/go/+/471601
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>