sbcl-2.6.9+: Use remove-duplicates in %prompt-args to avoid modifying a literal

https://github.com/atlas-engineer/nyxt/issues/3731

--- source/prompt-buffer.lisp.orig	2026-09-29 19:35:23 UTC
+++ source/prompt-buffer.lisp
@@ -709,7 +709,7 @@ See `update-prompt-input' to update the changes visual
        (error 'prompt-buffer-canceled)))))
 
 (sera:eval-always
-  (defvar %prompt-args (delete-duplicates
+  (defvar %prompt-args (remove-duplicates
                         (append
                          (public-initargs 'prompt-buffer)
                          (public-initargs 'prompter:prompter)