Build the ISO in CI

The Mac script needed hdiutil and diskutil. Apple tools. Now the disk image is made with mtools, no root, no loop devices, so it runs on Linux, on the Mac, and in a Docker container on a shared runner.

The runner fetches Terry's live CD from the Wayback Machine, checks the SHA-256 so nobody slips in a CIA one, boots it in QEMU without KVM and types MakeISO at it. TCG is slow. God is patient. Screenshots of every step are kept, so when it breaks you can see where the monk stopped typing.