wasm: use build script to detect wasm (#4865)

Co-authored-by: Taiki Endo <[email protected]>

tokio/src/runtime/blocking/pool.rs

@@ -125,7 +125,7 @@ const KEEP_ALIVE: Duration = Duration::from_secs(10);
 /// Tasks will be scheduled as non-mandatory, meaning they may not get executed
 /// in case of runtime shutdown.
 #[track_caller]
-#[cfg_attr(target_os = "wasi", allow(dead_code))]
+#[cfg_attr(tokio_wasi, allow(dead_code))]
 pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
 where
     F: FnOnce() -> R + Send + 'static,