pybridge: Fix cockpit_Machines directory lookup

lookup_config() must be called with a relative path, as otherwise
joining some "/etc/.." prefix to the given value always results in the
value itself, due to how path joining works. That led to trying to write
the config into the nonexisting /machines.d/ directory, breaking a lot
of TestMultiMachine tests on CoreOS with the Python bridge.

src/cockpit/internal_endpoints.py

@@ -124,7 +124,7 @@ def do_identity_changed(self, fd: Optional[int], errno: Optional[int]) -> None:
         self.notify()
 
     def __init__(self):
-        self.path = config.lookup_config('/machines.d')
+        self.path = config.lookup_config('machines.d')
         self.loop = asyncio.get_running_loop()
 
         # ignore the first callback