Organize downloads by prioritizing the platform, then the architecture. (#7900)

## Summary

PythonDownloadKey (cpython-3.13.0rc3-darwin-aarch64-none) and
PlatformTriple in `fetch-download-metadata.py` have a slight
inconsistency in the ordering of `os` and `arch`. In PythonDownloadKey,
`os` precedes `arch`, while in PlatformTriple, `arch` comes before
`platform` (equivalent to os). This difference in ordering affects the
sorting logic, giving arch higher priority than platform in the
`download-metadata.json` file, leading to a little bit of unexpected
order of entries.

Before:
<img width="676" alt="image"
src="https://github.com/user-attachments/assets/adb24a2e-da70-4a09-a702-4b5d71600b2c">
After:
<img width="725" alt="image"
src="https://github.com/user-attachments/assets/c6c76e6a-d3fd-43dc-bfb0-b3a4a3fe2b6b">

Author: j178