pythonPackages.tkinter: patch rpath to use only one interpreter

The `tkinter` module is copied from a build of `python` with
`x11Support=true;` but has a reference to that build of `python`. We
however want to use the module in combination with a build of `python`
with `x11Support=false;` (the default). Therefore we patch the rpath to
refer to that `python` instead.