Copy the overriding settings instead of a env var

binder/postBuild

@@ -5,7 +5,9 @@ set -e
 # NB: If you want to use Jupytext on your binder, don't install it from source,
 # just add "jupytext" to your "binder/requirements.txt" instead.
 BUILD_JUPYTERLAB_EXTENSION=1 pip install .[myst]
-export JUPYTER_CONFIG_PATH=${PWD}/binder:${JUPYTER_CONFIG_PATH}
+
+mkdir -p ${HOME}/.jupyter/labconfig
+cp binder/labconfig/* ${HOME}/.jupyter/labconfig
 
 # Create the notebook for our jupytext demo
 jupytext demo/get_started.md --to ipynb --update-metadata '{"jupytext":null}'