Split the aports-build job into arch-specific build-x86_64,
build-aarch64, ... jobs. Extend build_changed_aports.py to accept the
architecture as argument, and to build all packages for that arch where
possible.
Import and use pmbootstrap code for APKBUILD parsing etc, as we do this
already in various testcases running in this repository.
Co-Authored-By: Bart Ribbers <bribbers@disroot.org>
