allow replacing extensions index with environment variable

modules/ui_extensions.py

 import json
-import os.path
+import os
 import threading
 import time
 from datetime import datetime
@@ -564,7 +564,8 @@ def create_ui():
             with gr.TabItem("Available", id="available"):
                 with gr.Row():
                     refresh_available_extensions_button = gr.Button(value="Load from:", variant="primary")
-                    available_extensions_index = gr.Text(value="https://raw.githubusercontent.com/AUTOMATIC1111/stable-diffusion-webui-extensions/master/index.json", label="Extension index URL").style(container=False)
+                    extensions_index_url = os.environ.get('WEBUI_EXTENSIONS_INDEX', "https://raw.githubusercontent.com/AUTOMATIC1111/stable-diffusion-webui-extensions/master/index.json")
+                    available_extensions_index = gr.Text(value=extensions_index_url, label="Extension index URL").style(container=False)
                     extension_to_install = gr.Text(elem_id="extension_to_install", visible=False)
                     install_extension_button = gr.Button(elem_id="install_extension_button", visible=False)