systemd: make ctrl-alt-del target configurable. (#16911)

We currently only allow upstream's default of "reboot.target" due to the
way the symlinks are initialized. I made this configurable similar to the
default unit.

nixos/modules/system/boot/systemd-lib.nix

@@ -176,7 +176,7 @@ rec {
       ${optionalString (type == "system") ''
         # Stupid misc. symlinks.
         ln -s ${cfg.defaultUnit} $out/default.target
-
+        ln -s ${cfg.ctrlAltDelUnit} $out/ctrl-alt-del.target
         ln -s rescue.target $out/kbrequest.target
 
         mkdir -p $out/getty.target.wants/