ARROW-13134: [C++][CI] Unpin conda package for aws-sdk-cpp
ARROW-13134: [C++][CI] Unpin conda package for aws-sdk-cpp

Closes #11401 from pitrou/ARROW-13134-aws-sdk-unpin

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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