Disable backtrace test until converted to provider API

Fails to build starting on nightly-2022-08-03.

    error[E0599]: no method named `backtrace` found for struct `anyhow::Error` in the current scope
      --> tests/test_backtrace.rs:12:19
       |
    12 |     let _ = error.backtrace();
       |                   ^^^^^^^^^ method not found in `anyhow::Error`

tests/test_backtrace.rs

@@ -1,3 +1,7 @@
+// FIXME: needs to be updated to provide_ref/request_ref API.
+// https://github.com/rust-lang/rust/pull/99431
+#![cfg(any())]
+
 #[rustversion::not(nightly)]
 #[ignore]
 #[test]