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ARROW-12810: [Python] Stop AWS SDK from looking for metadata service

This significantly speeds up Python tests.

I opted to put it in Python instead of in the shell script so that interactive local development also benefits.

Closes #10348 from lidavidm/arrow-12810

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>