fix KOPS_AWS_ROLE_ARN assume behaviour

[zetaab]

without this change we got error:

(we have KOPS_AWS_ROLE_ARN=arn:aws:iam::xx:role/yy env defined`

% ./kops export kubecfg --admin=87600h
Error: got an error while querying for valid regions (verify your AWS credentials?): operation error EC2: DescribeRegions, https response error StatusCode: 403, RequestID: bee08acb-624f-460d-9a9d-5da92325b6d1, api error UnauthorizedOperation: You are not authorized to perform this operation. User: arn:aws:iam::xx:user/xx is not authorized to perform: ec2:DescribeRegions because no identity-based policy allows the ec2:DescribeRegions action

after this change it works fine

% ./kops export kubecfg --admin=87600h     
kOps has set your kubectl context to yyy.k8s.local

[k8s-ci-robot]

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