Specify ${pkgs.buxybox} for cp in the minecraft entrypoint

The binary was previously not included in the image, causing the
server properties to be misdefined.

etc/nixos/services/minecraft.nix

@@ -6,7 +6,7 @@ let
                            ${pkgs.busybox}/bin/mkdir -p /var/lib/
                            ${pkgs.gzip}/bin/gzip -dc ${./configs/minecraft.tar.gz} | ${pkgs.gnutar}/bin/tar -xf - -C /var/lib
                            echo 'eula=true' > /var/lib/minecraft/eula.txt
-                           cp -f ${properties} /var/lib/minecraft
+                           ${pkgs.busybox}/bin/cp -f ${properties} /var/lib/minecraft
                            $@
                          '';
   ops = pkgs.writeText "ops.json" (builtins.toJSON [