Use correct runtime path for Flatpak

See #6300

Independent from the implementation on master, due to the circular
import workaround.

qutebrowser/utils/standarddir.py

@@ -232,7 +232,14 @@ def _init_runtime(args: Optional[argparse.Namespace]) -> None:
         # Unfortunately this path could get too long for sockets (which have a
         # maximum length of 104 chars), so we don't add the username here...
 
-    _create(path)
+    from qutebrowser.utils import version
+    if version.is_sandboxed():
+        *parts, app_name = os.path.split(path)
+        assert app_name == APPNAME, app_name
+        path = os.path.join(*parts, 'app', os.environ['FLATPAK_ID'])
+    else:
+        _create(path)
+
     _locations[_Location.runtime] = path