Add cross-compiling support to generate-conda-packages GitHub Action and generate packages in robotology channel for linux-arm64 architectures

[traversaro]

At first, I would like to avoid that, and just ask people that wanted to have their packages for this architecture to just move their packages recipes to conda-forge (see #752). However, as osx-arm64 becomes a mainstream architecture, documenting this difference w.r.t. to Intel-based macOS would simply become more complex to document that to simply add support for osx-arm64 in the generate-conda-packages GitHub Action via cross-compilation. B.t.w, a nice side effect of this is that we would be able to also generate linux-arm64 packages even if GitHub Actions does not support arm64 Linux workers.

However, on the osx-64 we are currently blocked until all the dependencies are available, see #916 .

[traversaro]

fyi @Nicogene @ilaria-carlini

[traversaro]

Related instructions: https://conda-forge.org/blog/posts/2020-10-29-macos-arm64/#building-locally .

[traversaro]

As there is now native GitHub Actions support for osx-arm64, we do not need anymore to do cross-compilation for osx-arm64, I updated the title of the issue.