nix-gc.nix: prefer "nix-store" over "nix-collect-garbage" because the latter supports "--max-freed"

Works around https://github.com/NixOS/nix/issues/609.

Changed files
+1 -1
nixos
modules
services
misc
+1 -1
nixos/modules/services/misc/nix-gc.nix
···
systemd.services.nix-gc =
{ description = "Nix Garbage Collector";
-
script = "exec ${config.nix.package}/bin/nix-collect-garbage ${cfg.options}";
+
script = "exec ${config.nix.package}/bin/nix-store --gc ${cfg.options}";
startAt = optionalString cfg.automatic cfg.dates;
};