fix: use best_platform to verify the run platform

[ruben-arts]

I believe this allows osx-64 only environments' tasks to run on osx-arm64. @wolfv Could you verify. I only checked with faking the current platform on my machine.

e.g. this should work on osx-arm64:

[project]
name = "foo"
channels = ["foo"]
platforms = ["linux-64", "win-64", "osx-64"]

[tasks]
test = "echo test"

pixi run test

Fixes #1404

[wolfv]

I tested, and it works!