helpful-set-variable-function defaults to setopt if it's available, setq
otherwise. `setopt` is superior for this use case because it uses
custom.el's mechanism for setting variables (thus respecting custom.el's
setters and type checks), however, it is unavailable in Emacs 28 and
earlier (`setopt` was introduced in 29).
Doom defines `setq!`, which is a drop-in replacement for setopt, and is
functional in Emacs 28, so it is the better option here.
Ref: #7527
1bc5f441f0