message: when xargo is manually specified, don't try to upgrade it
diff: @@ -259,6 +259,10 @@ fn setup(ask_user: bool) {
  // First, we need xargo.
  if xargo_version().map_or(true, |v| v < (0, 3, 16)) {
  + if std::env::var("XARGO").is_ok() {
  + // The user manually gave us a xargo binary; don't do anything automatically.
  + show_error(format!("Your xargo is too old; please upgrade to the latest version"))
  + }
  let mut cmd = cargo();
  cmd.args(&["install", "xargo", "-f"]);
  ask_to_run(cmd, ask_user, "install a recent enough xargo");
  