Install on platform missing in the manifest

[ruben-arts]

Problem description

Pixi doesn't install on a platform not defined in it's project.platforms field.
This is by design, as we only create a lockfile for the platforms defined there.

This proposal tries to rethink that design.

Why change this?

• Not defining a platform in the manifest might not mean that the project doesn't work on that platform, but simply that it's not tested there.
• If the platform is missing, we also can't use that platform to solve the pypi-dependencies, which might be incorrect and thus limiting the use of pixi. E.g. Pixi cannot resolve pypi dependency on a mac #3081
• The overload of listing all platforms that do work grows the lockfile significantly, while you might not be interested to support that in your project by default.

Proposed implementation

• If the current platform is not defined, pixi does solve, install and run it.
• It will not put the solve result in the same lockfile but in a separate file that is not automatically committed, e.g. in the prefix or in the .pixi folder.
• Error's and warnings, take into account that it was not part of the intended list of platforms.
• platforms stays a default addition to the manifest, but is not a mandatory field anymore.

Use cases

• This will solve that issue where you can't solve pypi-dependencies on a platform that doesn't allow you to install the prefix on. (Not a fix if it actually can't install, but already an improvement over the current case)
• Not defining platforms will not create a lockfile for every platform when running pixi, goes against the vision of reproducibility but allows being a more flexible as we've seen multiple projects adding the pixi.lock to the .gitignore

Additional notes

• pixi lock --platform might become a tool to lock for specific platforms.

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This idea has room for refinement, please share your thoughts!