installer: Don't run as root

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.

nixos/doc/manual/installation/installing.xml

@@ -29,13 +29,14 @@
   </para>
 
   <para>
-   You are logged-in automatically as <literal>root</literal>. (The
-   <literal>root</literal> user account has an empty password.)
+   You are logged-in automatically as <literal>nixos</literal>.
+   The <literal>nixos</literal> user account has an empty password so you
+   can use <command>sudo</command> without a password.
   </para>
 
   <para>
    If you downloaded the graphical ISO image, you can run <command>systemctl
-   start display-manager</command> to start KDE. If you want to continue on the
+   start display-manager</command> to start the desktop environment. If you want to continue on the
    terminal, you can use <command>loadkeys</command> to switch to your
    preferred keyboard layout. (We even provide neo2 via <command>loadkeys de
    neo</command>!)
@@ -65,9 +66,9 @@
 
    <para>
     If you would like to continue the installation from a different machine you
-    need to activate the SSH daemon via <literal>systemctl start
-    sshd</literal>. In order to be able to login you also need to set a
-    password for <literal>root</literal> using <literal>passwd</literal>.
+    need to activate the SSH daemon via <command>systemctl start
+    sshd</command>. You then must set a password for either <literal>root</literal> or
+    <literal>nixos</literal> with <command>passwd></command> to be able to login.
    </para>
   </section>
  </section>