alpine build reproducer

Downloading source aports

Use pull-apk-source.sh on host machine repo/name, eg:

pull-apk-source.sh main/lttng-ust
pull-apk-source.sh community/lttng-tools

Then add to the git repository before building, as it does not work in actions.

just-build.sh runs "abuild checksum" before building so the package tarball is downloaded and any new patches have their checksums added before building.