JUPYTER_PATH can be used to provide additional plugins for jupyter to
use. Currently, JUPYTER_PATH is being unconditionally overridden, so
extensions are not found, e.g:
```
$ JUPYTER_PATH="path to dir with labextensions/@jupyter-widgets" jupyter labextension list 2>&1 | grep widgets
```
Instead of setting JUPYTER_PATH, prepend to it, so JUPYTER_PATH will be
respected:
```
$ JUPYTER_PATH="path to dir with labextensions/@jupyter-widgets" jupyter labextension list 2>&1 | grep widgets
@jupyter-widgets/jupyterlab-manager v5.0.13 enabled OK (python, jupyterlab_widgets)
```