Fixes a regression introduced in
cd370ab1 whereby kernels other than
ipython would not be found because JUPYTER_PATH was unset.

* guix-kernel/jupyter-server.scm (jupyter-kernel-path): New procedure.
(find-kernel-by-name): Rewrite and search in (jupyter-kernel-path) by
default.
* guix-kernel/environ.scm (env-profile->str): Remove.
(make-new-environment): Set 'GUIX_PROFILE'.