feat(hicache): support file backend reading directory config form env. (sgl-project#8498)

Author: hzh0425

python/sglang/srt/mem_cache/hicache_storage.py

@@ -85,7 +85,7 @@ def exists(self, key: str) -> bool:
 class HiCacheFile(HiCacheStorage):
 
     def __init__(self, file_path: str = "/tmp/hicache"):
-        self.file_path = file_path
+        self.file_path = os.getenv("SGLANG_HICACHE_FILE_BACKEND_STORAGE_DIR", file_path)
         tp_rank = get_tensor_model_parallel_rank()
         tp_size = get_tensor_model_parallel_world_size()
         self.tp_suffix = f"_{tp_rank}_{tp_size}" if tp_size > 1 else ""