Ignore buggy nonstandard_macro_braces clippy lint

Clippy bug: rust-lang/rust-clippy#7422

    error: use of irregular braces for `vec!` macro
     --> demo-hack-impl/src/lib.rs:6:1
      |
    6 | #[proc_macro_hack]
      | ^^^^^^^^^^^^^^^^^^
      |
      = note: `-D clippy::nonstandard-macro-braces` implied by `-D clippy::all`
    help: consider writing `#[proc_macro_hack]`
     --> demo-hack-impl/src/lib.rs:6:1
      |
    6 | #[proc_macro_hack]
      | ^^^^^^^^^^^^^^^^^^
      = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#nonstandard_macro_braces
      = note: this error originates in the attribute macro `proc_macro_hack` (in Nightly builds, run with -Z macro-backtrace for more info)

demo-hack-impl/src/lib.rs

@@ -1,3 +1,8 @@
+#![allow(
+    // Clippy bug: https://github.com/rust-lang/rust-clippy/issues/7422
+    clippy::nonstandard_macro_braces,
+)]
+
 use proc_macro::TokenStream;
 use proc_macro_hack::proc_macro_hack;
 use quote::quote;