Doesn't currently build on MyBinder

Nobody has claimed this yet.

## Issue
**What is linked from launch badge in Readme doesn't currently build on MyBinder**

It seems it isn't properly seeing `package.json` for some reason..

## Steps to reproduce
Go to the main repo Readme page [here](https://github.com/matplotlib/ipympl#ipympl) and click the `launch binder` badge.

The end of a build example that fails is shown below (`...` on first line represents start of the log that is not shown):

```
...
Successfully installed ipympl-0.8.7 ipywidgets-7.7.5 jupyterlab-widgets-1.1.4 widgetsnbextension-3.6.4
Remote version of pip: 23.1.2
Local version of pip: 23.0.1
Was pip installed by pip? False
Removed build tracker: '/tmp/pip-build-tracker-zn9i3t0k'
/srv/conda/envs/notebook/lib/python3.10/site-packages/setuptools/config/setupcfg.py:516: SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.
warnings.warn(msg, warning_class)
Installing /home/jovyan/ipympl/labextension -> jupyter-matplotlib
Building extension in /home/jovyan/ipympl
An error occurred.
FileNotFoundError: [Errno 2] No such file or directory: '/home/jovyan/ipympl/package.json'
See the log file for details: /tmp/jupyterlab-debug-22xaku6u.log
Removing intermediate container 9dfca785e89e
The command '/bin/sh -c ./.binder/postBuild' returned a non-zero code: 1
```

Start from the Launch Binder badge in the repository README and inspect the .binder/postBuild command shown in the failure log. Reproduce the build and investigate why /home/jovyan/ipympl/package.json is missing when the JupyterLab extension is built. Done means the Binder build completes successfully from the linked badge.

Written by the indexing model from the issue text.