nixos/networking: Use type lines for localCommands

Using types.str doesn't work if you want to mkBefore/mkAfter across
different module definitions, because it only allows for one definition
for the same priority.

This is especially useful if you deploy Hetzner machines via NixOps,
because the physical specification already defines localCommands.

Signed-off-by: aszlig <[e-mail]>
(cherry picked from commit 97801380b05447e59e47ba97d93f6c6cfefd56e4)

nixos/modules/tasks/network-interfaces.nix

···
     };
 
     networking.localCommands = mkOption {
-      type = types.str;
+      type = types.lines;
       default = "";
       example = "text=anything; echo You can put $text here.";
       description = ''