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

Fixes golang/go#70891, 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.

go/packages/packages_test.go

@@ -3361,9 +3361,12 @@ func main() {
 `)
 	gopath := filepath.Join(dir, "gopath")
 
+	// See https://golang.org/issue/70891.
+	// OpenBSD hasn't a /proc filesystem. When running with a -trimpath
+	// built go, it will always fail without a GOROOT.
 	pkgs, err := packages.Load(&packages.Config{
 		Mode: packages.NeedName | packages.NeedTarget,
-		Env:  []string{"GOPATH=" + gopath, "GO111MODULE=off"},
+		Env:  []string{"GOPATH=" + gopath, "GO111MODULE=off", "GOROOT=" + os.Getenv("GOROOT")},
 	}, filepath.Join(gopath, "src", "..."))
 	if err != nil {
 		t.Fatal(err)