nixos/restic-rest-server: set FreeBind=true for socket...

in order to allow binding to not (yet) locally available
interfaces. man 5 systemd.socket says:

> For robustness reasons it is recommended to use this option
> whenever you bind a socket to a specific IP address.

nixos/modules/services/backup/restic-rest-server.nix

@@ -136,6 +136,7 @@ in
       wantedBy = [ "sockets.target" ];
       socketConfig = {
         ReusePort = true;
+        FreeBind = true;
       };
     };