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

It looks like the systemd-initrd variant of the systemd-shutdown test
(systemd-initrd-shutdown) did not actually enable the systemd-initrd and
so was just evaluating to the same store path before this change.

Changed files
+1
nixos
+1
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 = ''