nix-gc.nix: Revert 'prefer "nix-store" over "nix-collect-garbage" because the...

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

This reverts commit ab6c8643. Issue
https://github.com/NixOS/nix/issues/609 has been resolved, the new Nix
version is available after 86eaeb4c, and
using nix-collect-garbage has the advantage that the '-d' flag is
available, which nix-store doesn't have.