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.

0 files changed, 0 insertions, 0 deletions