GitHub Actions: DO NOT install libxml2 package via brew, because macOS already provides it

[codecov[bot]]

Codecov Report

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

Comparison is base (c480d71) 85.02% compared to head (93cf224) 85.02%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3861 +/- ## ======================================= Coverage 85.02% 85.02% ======================================= Files 228 228 Lines 53901 53901 ======================================= Hits 45832 45832 Misses 8069 8069 ```

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[masatake]

LGTM. Interesting approach.