Add Artix Linux rootfs bootstrap and build support

[SkidProxyForker]

Motivation

• Add support for Artix Linux as an alternative rootfs target so users can build shimboot images with a non-systemd distribution.
• Provide a reproducible bootstrap and in-chroot setup flow for both x86_64 and ARM64 architectures to integrate Artix into the existing build tooling.

Description

• Documented Artix support and usage in README.md and added example build_complete.sh usage sudo ./build_complete.sh dedede distro=artix.
• Added artix-bootstrap.sh which fetches and extracts Artix packages, prepares a minimal filesystem, and installs base packages for x86_64 and aarch64 architectures, with configurable repo URL and arch via -a/-r options.
• Extended build_complete.sh and build_rootfs.sh to accept distro=artix and to call the new bootstrap; build_rootfs.sh also updated assert_deps to include curl gawk xz zstd.
• Added rootfs/opt/setup_rootfs_artix.sh which runs inside the chroot to configure mirrors/keyrings, install desktop or custom packages, enable OpenRC services, create user accounts, and apply shimboot-specific tweaks.

Testing

• No automated tests were executed for these changes.

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