org-export: Do not use `always' not available in Emacs <28

* lisp/ox.el (org-export--generate-copy-script): Use explicit `lambda'
instead of `always', which is not yet available in all the supported
Emacs versions.

Reported-by: Kyle Meyer <kyle@kyleam.com>

lisp/ox.el

@@ -2693,7 +2693,7 @@ The function assumes BUFFER's major mode is `org-mode'."
 	    (overlay-put (make-overlay start end) 'invisible invis))
           ;; Never write the buffer copy to disk, despite
           ;; `buffer-file-name' not being nil.
-          (setq write-contents-functions (list #'always)))))))
+          (setq write-contents-functions (list (lambda (&rest _) t))))))))
 
 (defun org-export--delete-comment-trees ()
   "Delete commented trees and commented inlinetasks in the buffer.