ci: split arm64 and amd64 build steps for faster compilation

[thalesmg]

Main build: https://github.com/emqx/emqx-builder/actions/runs/6584782118

[id]

I do not think this will work, docker image names do not contain architecture, they will conflict with each other:

- uses: docker/metadata-action@v4
  id: base_meta
  with:
    images: ghcr.io/${{ github.repository }}/base-${{ matrix.base_image_vsn }}
    tags: type=raw,value=${{ matrix.platform[0] }}

Here in the image tag we put matrix.platform[0] which is OS name only, like ubuntu22.04.

In fact, I'm not sure how this will work exactly. Best guess - last write wins. I.e. if arm64 build finishes after amd64, the resulting image will be the one for arm64 architecture.

To make use of arm64 runners we need something like this: https://docs.docker.com/build/ci/github-actions/multi-platform/#distribute-build-across-multiple-runners

[thalesmg]

ubuntu16.04 is not present on the public.ecr.aws mirror, so I had to leave it in docker hub.

[thalesmg]

after this is merged, I'll tag 5.2-3 and update the builder image in emqx

[id]

💯