CI: Update the publish and release workflows for multi-platform images

[larskanis]

• Add the build jobs before the pushlish and release jobs
• Simplify the Rakefile a little bit, by removing non-native builds
  • only release tasks are multi-platform now and should be used by adding the cache from non-native build host
• Fix aarch64-mingw-ucrt to use $TARGETPLATFORM for download selection

[flavorjones]

I'm sure you saw it already, but there is a build failure on the publish pipeline: https://github.com/rake-compiler/rake-compiler-dock/actions/runs/16400289294/job/46343040974#step:8:2399

[larskanis]

Thanks Mike! No idea why pushing aarch64-mingw-ucrt is the only job that fails currently, but the rest is already looking good. I'm happy that the images are available as x86_64 and arm64 images now! So far I had to build the images on my own ARM system, to make use of it on that system.

I also like the way it works: The Rakefile is not much more complex than before, with build and release tasks using the same code. Building runs on x86_64 runners and arm64 runners in parallel and the cache is used as glue to put outputs together into a multi-platform image.