Use app.config_file_path to find server extensions

This property in the base class already has the same logic to add config_dir to jupyter_config_path

JupyterHub overrides this to have slightly different logic, and the same should apply here

jupyter_server/serverapp.py

@@ -104,7 +104,6 @@
     base_flags,
     base_aliases,
 )
-from jupyter_core.paths import jupyter_config_path
 from jupyter_client import KernelManager
 from jupyter_client.kernelspec import KernelSpecManager
 from jupyter_client.session import Session
@@ -2154,11 +2153,7 @@ def find_server_extensions(self):
         # This enables merging on keys, which we want for extension enabling.
         # Regular config loading only merges at the class level,
         # so each level clobbers the previous.
-        config_paths = jupyter_config_path()
-        if self.config_dir not in config_paths:
-            # add self.config_dir to the front, if set manually
-            config_paths.insert(0, self.config_dir)
-        manager = ExtensionConfigManager(read_config_path=config_paths)
+        manager = ExtensionConfigManager(read_config_path=self.config_file_paths)
         extensions = manager.get_jpserver_extensions()
 
         for modulename, enabled in sorted(extensions.items()):