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ARROW-10202: [CI][Windows] Use sf.net mirror for MSYS2

Because repo.msys2.org is still down.

Closes #8373 from kou/ci-msys2-use-mirror

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