nixos/displayManagers/auto: allow root auto-login

The switch from slim to lightdm in #30890 broke some nixos tests
because lightdm by default doesn't permit auto-login for root.
Override /etc/pam.d/lightdm-autologin to allow it.

nixos/modules/services/x11/display-managers/auto.nix

@@ -49,6 +49,20 @@ in
       };
     };
 
+    # lightdm by default doesn't allow auto login for root, which is
+    # required by some nixos tests. Override it here.
+    security.pam.services.lightdm-autologin.text = lib.mkForce ''
+        auth     requisite pam_nologin.so
+        auth     required  pam_succeed_if.so quiet
+        auth     required  pam_permit.so
+
+        account  include   lightdm
+
+        password include   lightdm
+
+        session  include   lightdm
+    '';
+
   };
 
 }