make-disk-image: Reduce forking
make-disk-image: Reduce forking

The disk image calculator was using find + exec forking du for every
file in the disk image, making it very slow. Change du to accept files,
nul delimeted on stdin to speed it back up.

Before change:
nix-build nixos/tests/image-contents.nix  9.71s user 1.06s system 8% cpu 2:13.11 total

After change:
nix-build nixos/tests/image-contents.nix  9.93s user 1.23s system 21% cpu 51.601 total

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