push /task/cache_actor_icon at cache_actor_icon()

Microblog.pub sometimes writes error log as follows:
```
[ERROR] cache not available for (Icon's Path)/50/Kind.ACTOR_ICON
```

(Maybe) for the sake of caching actor's icon, `_cache_actor_icon()`
in `core/activitypub.py` runs `Tasks.cache_actor_icon()`.
However, `cache_actor_icon()` in `core/tasks.py` does not run
`/tasks/cache_actor_icon`.

core/tasks.py

@@ -35,6 +35,8 @@ class Tasks:
         if MEDIA_CACHE.is_actor_icon_cached(icon_url):
             return None
 
+        p.push({"icon_url": icon_url, "actor_iri": actor_iri}, "/task/cache_actor_icon")
+
     @staticmethod
     def cache_emoji(url: str, iri: str) -> None:
         if MEDIA_CACHE.is_emoji_cached(iri):