Fix display-buffer-override-next-command (bug#71654)

Fix display-buffer-override-next-command (bug#71654)

* lisp/window.el (display-buffer-override-next-command):
display-buffer-overriding-action's car can be either a function or a
list of functions. When a function,
(push action (car display-buffer-overriding-action)) will create a
cons-cell (action . function) instead of a list. In the clear-fun
callback, trying to
(delq action (car display-buffer-overriding-action)) causes an
wrong-type-argument error.

Copyright-paperwork-exempt: yes