message: Remove Diagnostic::backtrace
  Summary:
  `backtrace` [removed in latest rust](https://github.com/rust-lang/rust/commit/b2bbca393384c6d7e343decef5a927e16f003038).
  Remove from `Diagnostic` to [fix GitHub CI](https://github.com/facebookexperimental/starlark-rust/runs/7682156927?check_suite_focus=true).
diff: @@ -129,9 +129,10 @@ impl Error for Diagnostic {
  None
  }
  - fn backtrace(&self) -> Option<&std::backtrace::Backtrace> {
  - Some(self.message.backtrace())
  - }
  + // TODO(nga): figure out how to do it with unstable rust.
  + // fn backtrace(&self) -> Option<&std::backtrace::Backtrace> {
  + // Some(self.message.backtrace())
  + // }
  }
  impl Diagnostic {
  