Fix base_scores for sklearn GradientBoostingClassifier

[hcho3]

• Also update dev/prepare_pypi_release.py to download aarch64 wheels

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (10b6137) 83.73% compared to head (9545782) 83.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release_4.0 #536 +/- ## =============================================== - Coverage 83.73% 83.71% -0.02% =============================================== Files 70 70 Lines 6062 6062 Branches 510 510 =============================================== - Hits 5076 5075 -1 - Misses 986 987 +1 ```

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