luigi: enable local modules discovery

Currently, local module discovery is broken with luigi installed from
this derivation.

In the documentation, you can see the following command line syntax
used:

```
luigi --module module_name ...
```

However, currently this will result in an error:

```
ModuleNotFoundError: No module named 'module_name'
```

However, if the call is prepended with this:

```
PYTHONPATH=.:$PYTHONPATH
```

then it will work as expected.

This patch makes this the default behaviour.

pkgs/applications/networking/cluster/luigi/default.nix

@@ -19,6 +19,9 @@ python3Packages.buildPythonApplication rec {
   # Requires tox, hadoop, and google cloud
   doCheck = false;
 
+  # This enables accessing modules stored in cwd
+  makeWrapperArgs = ["--prefix PYTHONPATH . :"];
+
   meta = with lib; {
     homepage = https://github.com/spotify/luigi;
     description = "Python package that helps you build complex pipelines of batch jobs";