disable alloc/benches in Miri

Author: RalfJung

library/alloc/benches/lib.rs

@@ -1,6 +1,8 @@
 // Disabling on android for the time being
 // See https://github.com/rust-lang/rust/issues/73535#event-3477699747
 #![cfg(not(target_os = "android"))]
+// Disabling in Miri as these would take too long anyway.
+#![cfg(not(miri))]
 #![feature(btree_extract_if)]
 #![feature(iter_next_chunk)]
 #![feature(repr_simd)]