Support nixpkgs 20.03

[balsoft]

With nixpkgs 20.03, mix-to-nix breaks: it spits out errors like this:

  tar: _xd21wj8vkdm5n92cyy3v7c9wfmr7kyhg-bunt-0.2.0.mix.extracted/1E8.gz: Not found in archive
  tar: Exiting with failure status due to previous errors

It can be mitigated with a diff like this: serokell/mix-to-nix@83807e0 (build succeeds, everything works as expected) but I'm sure there is an actual solution that isn't me taking a stab in the dark.