cmd/go: add more info in t.Fatal message

Updates #20284

Change-Id: I4d55d6adec4e8efb54285ca27bb4fa39c76582c3
Reviewed-on: https://go-review.googlesource.com/46719Reviewed-by: Chris Broadfoot <cbro@golang.org>

src/cmd/go/go_test.go

@@ -4044,7 +4044,7 @@ func TestExecutableGOROOT(t *testing.T) {
 		cmd.Env = env
 		out, err := cmd.CombinedOutput()
 		if err != nil {
-			t.Fatal(err)
+			t.Fatalf("%s env GOROOT: %v, %s", exe, err, out)
 		}
 		goroot, err := filepath.EvalSymlinks(strings.TrimSpace(string(out)))
 		if err != nil {