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ARROW-16995: [CI][C++][MinGW]  Don't cache site-packages (#13534)

It may revert updated Python related MSYS2 packages such as
python-numpy. This caches built wheels instead.

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>