Disable backtrace test on miri

Starting in the most recent release of miri, the build.rs probe fails to
build even on nightly toolchain with one of the following errors:

    error: miri can only run programs that have a main function

    error: the current sysroot was built without `-Zalways-encode-mir`,
    or libcore seems missing. Use `cargo miri setup` to prepare a
    sysroot that is suitable for Miri.

which causes anyhow::Error's backtrace() inherent method to not exist.

    error[E0658]: use of unstable library feature 'backtrace'
      --> tests/test_backtrace.rs:20:19
       |
    20 |     let _ = error.backtrace();
       |                   ^^^^^^^^^
       |
       = note: see issue 53487 <rust-lang/rust#53487> for more information
       = help: add `#![feature(backtrace)]` to the crate attributes to enable

Author: dtolnay

tests/test_backtrace.rs

@@ -1,3 +1,5 @@
+#![cfg(not(miri))]
+
 #[rustversion::not(nightly)]
 #[ignore]
 #[test]