cmd/go: place GOROOT/bin at the beginning of PATH in 'go generate' and 'go test'

cmd/go: place GOROOT/bin at the beginning of PATH in 'go generate' and 'go test'

This causes tests and generators that execute 'go' as a subprocess to
use the same go command as the parent 'go test' or 'go generate'
command.

For #51473.

Change-Id: I003cf1d05d1c93a26c6a7fdfad25e86c11765f59
Reviewed-on: https://go-review.googlesource.com/c/go/+/404134
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>