store: Add another missing buffer flush.

This is a followup to 3809824199.

* guix/store.scm (add-file-tree-to-store): Add 'write-buffered-output'
call.

guix/store.scm

@@ -1044,6 +1044,7 @@ (define cache
                            #:file-port file-port
                            #:symlink-target symlink-target
                            #:directory-entries directory-entries)
+          (write-buffered-output server)
           (let loop ((done? (process-stderr server)))
             (or done? (loop (process-stderr server))))
           (let ((result (read-store-path port)))