* guix-jupyter-kernel.scm (%inferior-property): New variable.
(proxy-state-inferior, set-proxy-state-inferior)
(ensure-proxy-state-inferior): New procedures.
(%inferior): Remove.
(create-environment): Call 'proxy-state-inferior' instead of refering to
%INFERIOR.
(reply-execute-request): Call 'ensure-proxy-state-inferior' before
calling 'create-environment'.
(reply-complete-request): Call 'ensure-proxy-state-inferior' before
calling 'inferior-available-packages'.  Return the new state.