Ignore unexpected_cfgs lint for generated code

```
error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/is_none.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
  |
  = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `test`, `ub_checks`, `unix`, `windows`
  = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(rustfmt)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
  = note: `-D unexpected-cfgs` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`

error: unexpected `cfg` condition name: `rustfmt`
 --> src/gen/de.rs:6:13
  |
6 | #![cfg_attr(rustfmt, rustfmt::skip)]
  |             ^^^^^^^
  |
  = help: consider using a Cargo feature instead or adding `println!("cargo::rustc-check-cfg=cfg(rustfmt)");` to the top of the `build.rs`
  = note: see <https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-check-cfg> for more information about checking conditional configuration
```

Due to rust-lang/rust#124735, we cannot
disable this lint at module-level.

Author: taiki-e

src/lib.rs

@@ -50,6 +50,7 @@ See also the [`get` example](https://github.com/taiki-e/cargo-config2/blob/HEAD/
     clippy::std_instead_of_core,
 )]
 #![allow(clippy::must_use_candidate)]
+#![allow(unexpected_cfgs)] // https://github.com/rust-lang/rust/issues/124735
 
 // Refs:
 // - https://doc.rust-lang.org/nightly/cargo/reference/config.html

tools/codegen/src/file.rs

@@ -33,7 +33,9 @@ pub(crate) fn workspace_root() -> PathBuf {
 #[track_caller]
 pub(crate) fn header(function_name: &str) -> String {
     // rust-analyzer does not respect outer attribute (#[rustfmt::skip]) on
-    // a module without a body. So use inner attribute under cfg(rustfmt).
+    // a module without a body and unstable ignore option in .rustfmt.toml.
+    // https://github.com/rust-lang/rust-analyzer/issues/10826
+    // So use inner attribute under cfg(rustfmt).
     format!(
         "// SPDX-License-Identifier: Apache-2.0 OR MIT
 // This file is @generated by {bin_name}