CI: build packages for all supported arches

The new binary repository is rolled out (https://gitlab.com/postmarketOS/pmbootstrap/merge_requests/1835)!

I want to make maintaining the new binary repository as painless as possible. To fulfill that goal, we should only push to master (and other branches that we will build from in the future) what is known to build... for all arches.

Let's add CI checks to the pmaports.git merge requests for all arches.

TODO:

• add an architecture argument to build_changed_aports.py
• replace the existing build job with new build jobs for each supported architecture (x86_64, armhf, armv7, aarch64, x86). These call the above script, with the architecture as argument
• instead of directly building each changed package, the script should from now on check if the APKBUILD supports the desired architecture, and only then proceed with building for that arch. Otherwise print a message and skip the build.

@PureTryOut, you have been improving the CI stuff recently, would you like to take a shot at this? (or if somebody else would like to jump in, just drop a message here and get to it - I think this having this implemented will save us from a lot of work down the line and is one of the most important things that need to be improved for the new repo.)