require network connection for home-config clone

waiting for the network to be up with `network-online.target` will only work
with a NixOS 18.09 build after this commit:
<NixOS/nixpkgs@022eb6a>

you have to build your own image from that, as currently the latest 18.09
release binary is more than two months older. the issue behind this is
essentially inconsistent service names:
<NixOS/nixpkgs#59603>

here is how to build an image:
<https://nixos.org/nixos/manual/index.html#sec-building-cd>

this is what you get if you want to use immature software...

modules/home-config.nix

@@ -58,8 +58,8 @@ in
       nameValuePair ("home-config-${utils.escapeSystemdPath username}") {
         description = "initial home-manager configuration for ${username}";
         wantedBy = [ "multi-user.target" ];
-        wants = [ "nix-daemon.socket" ];
-        after = [ "nix-daemon.socket" ];
+        after = [ "nix-daemon.socket" "network-online.target" ];
+        requires = [ "nix-daemon.socket" "network-online.target" ];
         serviceConfig = with pkgs; {
           User = username;
           Type = "oneshot";