ci: fix aws s3 cmd on ubuntu 20.04

[crazy-max]

encounter an issue while merging #14685 linked to aws cli used on newest runners (since Ubuntu 20.04):

Run aws s3 sync --acl public-read _site s3://docs.docker.com-stage-us-east-1/ --delete

<botocore.awsrequest.AWSRequest object at 0x7f919fe3bfa0>
Error: Process completed with exit code 255.

we need to explicitly specify the region.

Signed-off-by: CrazyMax crazy-max@users.noreply.github.com

[thaJeztah]

LGTM

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