cmd/go: don't warn about GOROOT equal to GOPATH when both are the empty string
As of Go 1.19, runtime.GOROOT() reports the empty string if the binary
was built with -trimpath. cmd/go/internal/cfg uses the path of the go
command executable to reverse-engineer the correct GOROOT setting,
but that means that cmd/go's "GOPATH set to GOROOT" warning needs to
use cfg.GOROOT instead of runtime.GOROOT().
In addition, if we fail to find the GOROOT then there is no point in
complaining about GOPATH also being empty: the missing GOROOT will stop
everything right away anyway, so there is no point confusing the user
with an additional warning about GOPATH.
Updates #51461.
Updates #18678.
Updates #3207.
Change-Id: Id7d0f4dc2f229c202dfda4e6e8af5dea909bb16f
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/543955 Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>