tarsnap: fix license to mark as unfree
tarsnap: fix license to mark as unfree

tarsnap has always been unfree, but this wasn't expressed properly, so
it wouldn't be caught by allowUnfree = false.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs