Slim is abandoned and won't work with wayland.
It's in our best interest to use the display-manager
that makes most sense for Plasma5, sddm.
We've already moved on from it being default in #30890
and the graphical.nix profile, which the virtualbox profile uses,
has sddm anyway.
There's many reason why it is and is going to
continue to be difficult to do this:
1. All display-managers (excluding slim) default PAM rules
disallow root auto login.
2. We can't use wayland
3. We have to use system-wide pulseaudio
4. It could break applications in the session.
This happened to dolphin in plasma5
in the past.
This is a growing technical debt, let's just use
passwordless sudo.
Because when I see "config.system.build.manual.manual" after I forgot
what it means I ask "Why do I need that second `.manual` there again?".
Doesn't happen with `config.system.build.manual.manualHTML`.
