ARROW-13134: [C++][CI] Pin aws-sdk-cpp to < 1.9

Closes #10620 from pitrou/ARROW-13134

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

Author: pitrou

ci/conda_env_cpp.yml

@@ -15,7 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-aws-sdk-cpp
+aws-sdk-cpp<1.9
 benchmark=1.5.2
 boost-cpp>=1.68.0
 brotli