Use sys._xoptions to pass bundle_exe_dir

This allows it to be used with the make app kitty bundle as well.
This makes it robust against launching in environments where
python3 is not on PATH. See #1280

kitty/constants.py

@@ -22,7 +22,7 @@ WindowGeometry = namedtuple('WindowGeometry', 'left top right bottom xnum ynum')
 def kitty_exe():
     ans = getattr(kitty_exe, 'ans', None)
     if ans is None:
-        rpath = getattr(sys, 'bundle_exe_dir', None)
+        rpath = sys._xoptions.get('bundle_exe_dir')
         if not rpath:
             items = os.environ['PATH'].split(os.pathsep)
             seen = set()