message: Rename XARGO env var to XARGO_CHECK
  This reflects the fact that we want bootstrap to override `xargo-check`,
  not `xargo
diff: @@ -101,7 +101,7 @@ fn cargo() -> Command {
  }
  fn xargo() -> Command {
  - if let Ok(val) = std::env::var("XARGO") {
  + if let Ok(val) = std::env::var("XARGO_CHECK") {
  // Bootstrap tells us where to find xargo
  Command::new(val)
  } else {
  @@ -280,7 +280,7 @@ fn setup(ask_user: bool) {
  // First, we need xargo.
  if xargo_version().map_or(true, |v| v < XARGO_MIN_VERSION) {
  - if std::env::var("XARGO").is_ok() {
  + if std::env::var("XARGO_CHECK").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"))
  }
  