cmd/internal/browser: use xdg-open only from a desktop session

xdg-open's man page says:
> xdg-open is for use inside a desktop session only.

Use the DISPLAY environment variable to detect this.

Updates #19131.

Change-Id: I3926b3e1042393939b2ec6aacd9b63ac8192df3b
Reviewed-on: https://go-review.googlesource.com/37390Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/internal/browser/browser.go

@@ -23,7 +23,10 @@ func Commands() [][]string {
 	case "windows":
 		cmds = append(cmds, []string{"cmd", "/c", "start"})
 	default:
-		cmds = append(cmds, []string{"xdg-open"})
+		if os.Getenv("DISPLAY") != "" {
+			// xdg-open is only for use in a desktop environment.
+			cmds = append(cmds, []string{"xdg-open"})
+		}
 	}
 	cmds = append(cmds,
 		[]string{"chrome"},