check for --privhelper_path when deciding whether to use sudo

Summary:
When running the Eden tests using buck2, this flag is used because the
privhelper is not immediately next to the EdenFS binary.

This diff updates the approximation of the privhelper path introduced in
D31870880 (710f03501d) accordingly so that it also works in this case.

Reviewed By: xavierd

Differential Revision: D32070038

fbshipit-source-id: c111cc533bb9e9d111bfe9bad71e817655b3d613

eden/fs/cli/daemon.py

@@ -210,6 +210,16 @@ def prepare_edenfs_privileges(
         return (cmd, env)
 
     privhelper = os.path.join(os.path.dirname(daemon_binary), "edenfs_privhelper")
+
+    # EdenFS accepts a privhelper_path argument to locate the privhelper, so if
+    # we are about to pass that we should extract it.
+    next_arg_is_privhelper = False
+    for arg in cmd:
+        if next_arg_is_privhelper:
+            privhelper = arg
+            break
+        next_arg_is_privhelper = arg == "--privhelper_path"
+
     # If the EdenFS privhelper is installed as setuid root we don't need to use
     # sudo.
     try: