Add i386 and ARM autopkgtest support

This MR enables autopkgtest jobs for i386 in sid, forky, trixie, bookworm, bullseye, and their backports suites.

It simplifies the arm64 setup for unstable by removing ENABLE_ARM64_JOBS and switching the image to stable to match ci.d.n.

It also adds autopkgtest jobs for arm64, armel, and armhf in sid and trixie, since these are the most used suites

Part of: pipeline#364 (closed)