Use 'sxml->html-string' some more.

* guix-jupyter-kernel.scm (reply-for-environment): Use
'sxml->html-string'.

guix-jupyter-kernel.scm

@@ -134,10 +134,7 @@
   "Send KERNEL a reply to MESSAGE saying that we're preparing environment
 NAME with MANIFEST."
   (reply-html kernel message
-              (call-with-output-string
-                (lambda (port)
-                  (sxml->xml (manifest->shtml manifest name)
-                             port)))
+              (sxml->html-string (manifest->shtml manifest name))
               count))
 
 (define* (reply-for-environment-kernel kernel message