kernel: Allow downloads of local files.

Fixes #4.

* guix-jupyter-kernel.scm (handle-download): Handle the case where
'url-fetch' returns a plain file.

guix-jupyter-kernel.scm

@@ -363,9 +363,12 @@ HASH of type HASH-ALGO."
                                  (mlet %store-monad
                                      ((drv (url-fetch url (string->symbol hash-algo)
                                                       hash/bv file)))
-                                   (mbegin %store-monad
-                                     (built-derivations (list drv))
-                                     (return (derivation->output-path drv))))))
+                                   (if (derivation? drv)
+                                       (mbegin %store-monad
+                                         (built-derivations (list drv))
+                                         (return
+                                          (derivation->output-path drv)))
+                                       (return drv))))) ;plain file
                   (environment (proxy-state-default-environment state))
                   (target      (lookup-proxied environment state))
                   (home        (assq-ref (kernel-properties target) 'home)))