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/lib/make-disk-image.nix

@@ -278,7 +278,7 @@ let format' = format; in let
       additionalSpace=$(( $(numfmt --from=iec '${additionalSpace}') + reservedSpace ))
 
       # Compute required space in filesystem blocks
-      diskUsage=$(find . ! -type d -exec 'du' '--apparent-size' '--block-size' "${blockSize}" '{}' ';' | cut -f1 | sum_lines)
+      diskUsage=$(find . ! -type d -print0 | du --files0-from=- --apparent-size --block-size "${blockSize}" | cut -f1 | sum_lines)
       # Each inode takes space!
       numInodes=$(find . | wc -l)
       # Convert to bytes, inodes take two blocks each!