fix asset loader preregistration for multiple assets (#9453)

# Objective

fix #9452

when multiple assets are queued to a preregistered loader, only one gets
unblocked when the real loader is registered.

## Solution

i thought async_channel receivers worked like broadcast channels, but in
fact the notification is only received by a single receiver, so only a
single waiting asset is unblocked. close the sender instead so that all
blocked receivers are unblocked.

Author: robtfm

crates/bevy_asset/src/asset_server.rs

@@ -236,7 +236,7 @@ impl AssetServer {
             loaders[existing_index] = MaybeAssetLoader::Ready(Arc::new(loader));
             if let Some(sender) = maybe_sender {
                 // notify after replacing the loader
-                let _ = sender.send_blocking(());
+                let _ = sender.close();
             }
         } else {
             loaders.push(MaybeAssetLoader::Ready(Arc::new(loader)));