Merge pull request #217942 from lilyinstarlight/fix/nixos-test-systemd-initrd-shutdown

nixos/tests/systemd-shutdown: ensure systemd-initrd variant actually enables it

nixos/tests/systemd-shutdown.nix

···
     systemd.shutdownRamfs.contents."/etc/systemd/system-shutdown/shutdown-message".source = pkgs.writeShellScript "shutdown-message" ''
       echo "${msg}"
     '';
+    boot.initrd.systemd.enable = systemdStage1;
   };
 
   testScript = ''