Matrix can't be used due to build-push-action not supporting it

[suntorytimed]

docker/build-push-action#671 is currently blocking the usage of a matrix to separate the builds for each platform and increase the efficiency and speed of rebuilds. Once this is supported the GitHub Actions should be adjusted accordingly, as it saves 30 minutes on amd64 images and 20 minutes on the slowest riscv64 platform.

[suntorytimed]

reverted the pipeline split via matrix with 4f7361f