Support engine override, support overriding some env, and improve macOS compatibility

[dvicory]

With the latest Docker on Homebrew, build no longer works. It seems an engine override is a nice addition, in case you prefer to not select docker as the default choice.

Additionally, to allow forks to be more maintainable, allow the docker tag, repository, ZBM, and EFI to be changed through env. These are exposed through env since most users should not customize these.

Use the latest released ZBM as the default.

Improve macOS compatibility by using GNU versions for du and stat. inject (stat), and iso (du) are fixed by checking for GNU versions.

Before engine override/macOS fixes

builder with latest Docker

DEPRECATED: The legacy builder is deprecated and will be removed in a future release.
            Install the buildx component to build images with BuildKit:
            https://docs.docker.com/go/buildx/

unknown flag: --progress

inject

...
Secrets were injected, appending '_injected' to name
Copying original EFI ./output/zquickinit-2025-03-04_061842.efi to output location zquickinit-2025-03-04_061842_injected.efi...
Extracting initramfs from EFI ./output/zquickinit-2025-03-04_061842.efi to /var/folders/ml/7k_3hr454_n8sn0kslvmj8bm0000gn/T/tmp.az8PPgXf/zquickinit.img.zst
stat: illegal option -- c
usage: stat [-FLnq] [-f format | -l | -r | -s | -x] [-t timefmt] [file ...]

iso

Generating ISO to zquickinit.iso
No EFI UKI source specified  - searching for image...
Found EFI: ./zquickinit-2025-03-04_061842_injected.efi
du: unrecognized option `--apparent-size'
usage: du [-Aclnx] [-H | -L | -P] [-g | -h | -k | -m] [-a | -s | -d depth] [-B blocksize] [-I mask] [-t threshold] [file ...]
Generating raw file image for VM as /var/folders/ml/7k_3hr454_n8sn0kslvmj8bm0000gn/T/tmp.VFhJq9VJ/efs_partition.img
Disk full

[dvicory]

Fixing a bug with stat during Proxmox installation.

This has been fixed. I've tested installing in playground using the default options as well as converting to being encrypted.

This script is quite magnificent. Amazing work!

I do have another fix for an issue I encountered generating initramfs for zquick_sshd (not a regression here) - would you prefer a separate PR or for me to include that here?