Mark async worker tmp file as utf-8-emacs-unix (bug#48029)

* lisp/emacs-lisp/comp.el (comp-final): Mark async worker tmp file
as utf-8.
* test/src/comp-tests.el (48029-1): New test.
* test/src/comp-resources/comp-test-funcs.el
(comp-test-48029-nonascii-žžž-f): New function.

lisp/emacs-lisp/comp.el

@@ -3697,7 +3697,7 @@ Prepare every function for final compilation and drive the C back-end."
 				 (file-name-base output) "-")
 			 nil ".el")))
 	(with-temp-file temp-file
-          (insert ";; -*-coding: nil; -*-\n")
+          (insert ";; -*-coding: utf-8-emacs-unix; -*-\n")
           (mapc (lambda (e)
                   (insert (prin1-to-string e)))
                 expr))