go/packages: add GOROOT env to avoid TestTarget failure in OpenBSD

When running tests in OpenBSD if 'go' is built with -trimpath,
the TestTarget will always fail. Because when invoked without
proper environments, 'go' itself fails to find the GOROOT path.

Fixes golang/go#70891

Change-Id: I829b77686ae24d869653365f3e44e457c76b46bb
GitHub-Last-Rev: 6e1311b
GitHub-Pull-Request: #552
Reviewed-on: https://go-review.googlesource.com/c/tools/+/637961
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

go/packages/packages_test.go

@@ -3339,7 +3339,7 @@ func main() {
 
 	pkgs, err := packages.Load(&packages.Config{
 		Mode: packages.NeedName | packages.NeedTarget,
-		Env:  []string{"GOPATH=" + gopath, "GO111MODULE=off"},
+		Env:  append(os.Environ(), "GOPATH=" + gopath, "GO111MODULE=off"),
 	}, filepath.Join(gopath, "src", "..."))
 	if err != nil {
 		t.Fatal(err)