Explicitly install nodejs

Earlier, we were depending on it being a dependency
of jupyterlab - I don't think that works anymore.

repo2docker/buildpacks/conda/environment.yml

@@ -1,5 +1,6 @@
 dependencies:
   - python=3.7
+  - nodejs=14
   - ipywidgets==7.6.3
   - jupyter-offlinenotebook==0.2.1
   - jupyter-resource-usage==0.5.1