Prevent upgrade of `jupyterhub`

[chbrandt]

If we do a conda update conda, many things can happen, besides upgrading the conda manager itself... pip can be uninstalled (conda/conda#10887), other packages can be updated as well.

We don't want, though, to update jupyterhub because its version should match that of the server.
So, we have to pin it to whatever version is used.

Can be done like that:

(base) jovyan:/work$ echo "jupyterhub $JUPYTERHUB_VERSION" >> $CONDA_PREFIX/conda-meta/pinned 

or like that (the actual version installed...should be the same as JUPYTERHUB_VERSION):

(base) jovyan:/work$ echo "jupyterhub $(jupyterhub --version)" >> $CONDA_PREFIX/conda-meta/pinned 

Causing file pinned to be:

(base) jovyan:/work$ cat $CONDA_PREFIX/conda-meta/pinned 
python 3.10.9
jupyterhub 3.0.0
(base) jovyan:/work$ echo $CONDA_PREFIX/
/opt/conda/