Restore backpack support as soon as it will be supported by nix. #4
Description
Currently, it's not possible to use the cheops-logger library with nix, as it requires backpack (signatures) support to work with metrics. It seems that NixOS/nixpkgs#40128 needs to be sorted out first. Once it's sorted or if there is a workaround it would be possible to use the backpack again.
Once this issue is solved 0737ee0 should be reverted, but it will be a major version bump.