nixos system path: add rsync back

I was used anyway, and we did this on master
(I don't think 16.09 should be "ahead of" master). See:
https://github.com/NixOS/nixpkgs/commit/0aa75206705afc

nixos/modules/config/system-path.nix

@@ -34,6 +34,7 @@ let
       config.programs.ssh.package
       pkgs.perl
       pkgs.procps
+      pkgs.rsync # used by nixos-install anyway
       pkgs.strace
       pkgs.su
       pkgs.time