Replace PONOS_DATA_DIR with the existing PONOS_DATA

Ponos already provides the PONOS_DATA environment variable to point at /data, so we can remove PONOS_DATA_DIR.

arkindex_worker/worker/base.py

@@ -145,7 +145,7 @@ class BaseWorker(object):
             merge_parents_cache(
                 task["parents"],
                 self.cache_path,
-                data_dir=os.environ.get("PONOS_DATA_DIR", "/data"),
+                data_dir=os.environ.get("PONOS_DATA", "/data"),
                 chunk=os.environ.get("ARKINDEX_TASK_CHUNK"),
             )