onboard: move python dependencies to pythonPath

It is deprecated but otherwise wrapPython adds everything that is propagated, including dev outputs, to program_PATH, which ends up in the wrapper.

pkgs/applications/misc/onboard/default.nix

···
     xorg.libxkbfile
   ] ++ lib.optional atspiSupport at-spi2-core;
 
-  propagatedBuildInputs = with python3.pkgs; [
+  pythonPath = with python3.pkgs; [
     dbus-python
     distutils_extra
     pyatspi