fix(stasis): let the desktop suspend on idle again

moonarch 7a24cdb puts a suspend step back into the desktop plan; it was lost
when that plan stopped inheriting the ac: blocks. systemctl has to be on the
unit PATH for it, or the step fails silently like swaylock did.

modules/services.nix

@@ -189,6 +189,7 @@ in
       libnotify
       swaylock
       brightnessctl
+      systemd
     ];
     # No waylandClientHardening: swaylock is a child of this unit and
     # authenticates through the setuid-root unix_chkpwd. `systemd --user` builds