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.

pkgs/top-level/python-packages.nix

···
     disabled = isPy26 || isPyPy;
 
     installPhase = ''
+      # Move the tkinter module
       mkdir -p $out/${py.sitePackages}
-      ls -Al lib/${py.libPrefix}/lib-dynload/ | grep tkinter
       mv lib/${py.libPrefix}/lib-dynload/_tkinter* $out/${py.sitePackages}/
+      # Update the rpath to point to python without x11Support
+      old_rpath=$(patchelf --print-rpath $out/${py.sitePackages}/_tkinter*)
+      new_rpath=$(sed "s#${py}#${python}#g" <<< "$old_rpath" )
+      patchelf --set-rpath $new_rpath $out/${py.sitePackages}/_tkinter*
     '';
 
     inherit (py) meta;