feat: add platform override

[DaniAffCH]

Description

Adds support for the PIXI_OVERRIDE_PLATFORM environment variable, allowing users to install packages for a foreign architecture. It is useful for cross-compilation workflows where target dependencies need to be present on the host machine.

As suggested in #5918, I went with an env var rather than a new CLI flag.

A few design choices worth mentioning:

• If the value doesn't match a valid Platform, pixi raises a warning panics. Silently falling back to the host platform felt wrong since the user clearly has a different intention.
• The emulation warning is skipped when the override is set. To my understanding, the warning exists to notify users of an automatic fallback they didn't ask for. With an explicit override, there's no implicit fallback, so the warning doesn't apply.

Note: I'm not familiar with Rust at all, so feel free to point out if something can be improved!
Fixes #5918

How Has This Been Tested?

Added two unit tests: test_best_platform_override_env_var and test_best_platform_override_invalid_value.
I also manually tested it on a linux-64 host by running:

PIXI_OVERRIDE_PLATFORM=linux-aarch64 pixi install

and verified the installed libraries are aarch64.

AI Disclosure

This PR doesn't contain AI-generated code.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.

[baszalmstra]

Thanks for this!

Lets just return a proper warning instead of panicking!

[DaniAffCH]

Lets just return a proper warning instead of panicking!

Done! Now it raises a warning and falls back on the local platform. Thanks for your quick review

[ruben-arts]

Added docs, looks good to me now.