feat: use best_platform in a few places for cross-platform running

[wolfv]

This is a draft. It adds a best_platform to most actions.

The idea is that we allow the selection of another platform than the current one for "cross-platform" running. For example, Apple has implemented pretty good x86 emulation via Rosetta which means that most osx-64 packages run very well on osx-arm64.

If a pixi.toml does not support osx-arm64 (yet) we allow to fall back to osx-64.

This is especially important for bioconda as they are currently not building any packages for osx-arm64.

[wolfv]

fixes: #875

cc @corneliusroemer would be interested in your thoughts.

[pavelzw]

How about a small warning when the platform doesn't match your system platform?

[wolfv]

Yes, agree with the warning.

I had another thought - we could also do a setting as in CONDA_SUBDIR=linux-64 where we would override the subdir per config / task / etc., e.g.

[platform."osx-arm64".options]
override_platform = "osx-64" # or platform_override?

But not sure if this is not too verbose.

Otherwise for configuration we would do somethign like:

platforms = ["osx-64", "osx-arm64:osx-64", "linux-*:linux-64"]

And map the platforms on each other like this.

[corneliusroemer]

I don't know enough about pixi to be of much help except for quite high-level feedback.

Fallback sounds good, warning too. Make sense to allow disabling fallback.

Happy to test drive if you have something I can run locally.

[nichmor]

looks very good to me :)

[ruben-arts]

This feature sounds logical but I would like some documentation and examples to try it out myself. As I can't get it to work from what I've been trying based on this pr.