The CPython Pixi packages are now all accessible at the same
``Tools/pixi-packages`` subdirectory, rather than at
``Tools/pixi-packages/{variant}`` as before. Variants are now selected not
via subdirectory but via ``flags``; see
https://pixi.prefix.dev/latest/concepts/package_specifications/#extras-and-flags
for usage instructions. The ``tsan-freethreading`` variant has been renamed
to ``tsan_freethreading``, while the ``default``, ``asan``, and
``freethreading`` variants retain their previous names.
