title: Set python path in spacy project
body: <p>I'm using a spacy project to train a model.  I've registered a custom language so I need to provide some python code on the command line.  Here is a snippet of my project.yml</p>
  <pre><code>commands:
  
    - name: train
      script:
        - python -m spacy train configs/config.cfg --code my_language.py
  </code></pre>
  <p>The issue I'm having is how to specify the path to <code>my_language.py</code>.  I don't want to have to copy this code to my training directory (it is a Python package and not just one file).</p>
  <p>I tried updating my Python path in project.yml like this:</p>
  <pre><code>- export PYTHONPATH=... ; python -m spacy train configs/config.cfg --code my_language.py
  </code></pre>
  <p>but I get the error:</p>
  <pre><code>Can not execute command 'export PYTHONPATH=...
  </code></pre>
  <p>Any suggestions for how to do this?</p>