nixos-generate-config: do not build btrfs-tools when btrfs is not used
nixos-generate-config: do not build btrfs-tools when btrfs is not used

cross-compilation of `btrfs-tools` is broken, and this usually needless dependency of each system closure on `btrfs-tools` prevents cross-compilation of whole system closures
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