linkchecker: fix bad interaction with python3

Previously, `nix-shell -p linkchecker python3` would lead to a broken
`python3` command. That's because Python 2 packages would pollute the
environment and confuse the interpreter. `linkchecker` has propagated
build inputs. So the solution is to use `pythonPath` instead, to avoid
the pollution.

pkgs/tools/networking/linkchecker/default.nix

@@ -19,7 +19,7 @@ python2Packages.buildPythonApplication rec {
   version = "9.3.1";
 
   nativeBuildInputs = [ gettext ];
-  propagatedBuildInputs = (with python2Packages; [ 
+  pythonPath = (with python2Packages; [
     requests
   ]) ++ [ gettext ];