Fix mirror script to handle newer metadata format. (#10050)
## Summary
The architecture details in `crates/uv-python/download-metadata.json` is
now a dictionary with family and variant data, whereas it used to be a
string. This patches the architecture filter in
`scripts/create-python-mirror.py` to support both scenarios.
## Test Plan
Tested manually using `uv run ./scripts/create-python-mirror.py --name
cpython --arch x86_64 --os linux --from-all-history`
