org-attach: Make `org-attach-directory' a safe variable

* lisp/org-attach.el (org-attach-directory): Add :safe keyword.

lisp/org-attach.el

@@ -54,7 +54,8 @@
 If this is a relative path, it will be interpreted relative to the directory
 where the Org file lives."
   :group 'org-attach
-  :type 'directory)
+  :type 'directory
+  :safe #'stringp)
 
 (defcustom org-attach-commit t
   "If non-nil commit attachments with git.