emacs: Don't override the user's 'geiser-guile-load-path'.

* emacs/guix-backend.el (guix-start-repl): Add GUIX-LOAD-PATH to the
  front of GEISER-GUILE-LOAD-PATH instead of overriding it.

emacs/guix-backend.el

@@ -206,7 +206,8 @@ this address (it should be defined by
   ;; A mix of the code from `geiser-repl--start-repl' and
   ;; `geiser-repl--to-repl-buffer'.
   (let ((impl 'guile)
-        (geiser-guile-load-path (list guix-load-path))
+        (geiser-guile-load-path (cons guix-load-path
+                                      geiser-guile-load-path))
         (geiser-repl-startup-time guix-repl-startup-time))
     (with-current-buffer buffer
       (geiser-repl-mode)