Temporarily disable aarch64_be test

rust-lang/compiler-builtins#539

```
error: couldn't read /home/runner/work/setup-cross-toolchain-action/setup-cross-toolchain-action/rust-cross-toolchain/target/aarch64_be-unknown-linux-gnu/debug/build/compiler_builtins-3cff6482b464b138/out/outlined_atomics.rs: No such file or directory (os error 2)
   --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/compiler_builtins-0.1.95/src/aarch64_linux.rs:270:1
    |
270 | include!(concat!(env!("OUT_DIR"), "/outlined_atomics.rs"));
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: this error originates in the macro `include` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `compiler_builtins` (lib) due to previous error
```

Author: taiki-e

.github/workflows/ci.yml

@@ -48,7 +48,8 @@ jobs:
           # rustc --print target-list | grep -e '-linux-gnu'
           - aarch64-unknown-linux-gnu
           # - aarch64-unknown-linux-gnu_ilp32 # tier3
-          - aarch64_be-unknown-linux-gnu # tier3
+          # TODO: compiler-builtins bug https://github.com/rust-lang/compiler-builtins/pull/539
+          # - aarch64_be-unknown-linux-gnu # tier3
           # - aarch64_be-unknown-linux-gnu_ilp32 # tier3
           - arm-unknown-linux-gnueabi
           # - arm-unknown-linux-gnueabihf # supported in rust-cross-toolchain but not ported to this action