fix: remove erroneous async keyword from sync ActorClient.default_build (#620)

## Summary

- `ActorClient.default_build` (the **sync** variant) was incorrectly
declared as `async def` — a copy-paste error from the async variant
- Calling it returned a coroutine object instead of a `BuildClient`,
making it unusable without `await` (which makes no sense on the sync
client)
- Removed the `async` keyword so it behaves as a regular synchronous
method

## Test plan
- [ ] Verify existing unit tests pass
- [ ] Verify `ActorClient(...).default_build()` returns a `BuildClient`
directly (not a coroutine)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

src/apify_client/clients/resource_clients/actor.py

@@ -439,7 +439,7 @@ def runs(self) -> RunCollectionClient:
         """Retrieve a client for the runs of this Actor."""
         return RunCollectionClient(**self._sub_resource_init_options(resource_path='runs'))
 
-    async def default_build(
+    def default_build(
         self,
         *,
         wait_for_finish: int | None = None,