bug#64990: jupyter kernels install kernel.json without GUIX_PYTHONPATH

[Ricardo Wurmus <rekado@elephly.net>]

Consider this scenario: a number of users access a shared jupyterhub /
jupyterlab installation, which allows them to launch their own jupyter
kernels.  The shared installation was deployed with Guix.  The user
kernels are installed like this:

    $ guix shell python python-ipykernel python-foo python-bar …
    $ [env] python3 -m ipykernel install --user --name whatever --display-name whatever
    Installed kernelspec whatever in /home/rekado/.local/share/jupyter/kernels/whatever

The installed kernel.json looks like this:

--8<---------------cut here---------------start------------->8---
{
 "argv": [
  "/gnu/store/…-profile/bin/python3",
  "-m",
  "ipykernel_launcher",
  "-f",
  "{connection_file}"
 ],
 "display_name": "whatever",
 "language": "python",
 "metadata": {
  "debugger": true
 }
]
--8<---------------cut here---------------end--------------->8---

That’s not sufficient because the “ipykernel_launcher” module is
provided by the python-ipykernel package and the shared jupyter{hub,lab}
doesn’t know about this package.  That’s because it doesn’t use the
GUIX_PYTHONPATH of the profile containing the user’s packages.

Perhaps we could add a profile hook that adds an “env” field to the
generated kernel.json file, which augments the existing GUIX_PYTHONPATH
with the appropriate value for the current profile.

We can do this on the command line like this:

--8<---------------cut here---------------start------------->8---
  python3 -m ipykernel install \
    --user --name whatever --display-name whatever \
    --env GUIX_PYTHONPATH "${GUIX_PYTHONPATH}:\${GUIX_PYTHONPATH}"
--8<---------------cut here---------------end--------------->8---

Or we can edit the JSON directly.

The documentation of the kernel.json format is available here:
https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs

-- 
Ricardo