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

This reverts commit ab6c8643d451b04884e938cd7ed2186663d32a96. Issue
https://github.com/NixOS/nix/issues/609 has been resolved, the new Nix
version is available after 86eaeb4c0a31e623c01f0d39fd5b3e64ce5f80b5, and
using nix-collect-garbage has the advantage that the '-d' flag is
available, which nix-store doesn't have.

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-store --gc ${cfg.options}";
+
script = "exec ${config.nix.package}/bin/nix-collect-garbage ${cfg.options}";
startAt = optionalString cfg.automatic cfg.dates;
};