Add env var for cache file

modules/cache.py

 import json
+import os
 import os.path
 import threading
 import time
 
 from modules.paths import data_path, script_path
 
-cache_filename = os.path.join(data_path, "cache.json")
+cache_filename = os.environ.get('SD_WEBUI_CACHE_FILE', os.path.join(data_path, "cache.json"))
 cache_data = None
 cache_lock = threading.Lock()