Use sts get-caller-identity for whoami

[simonw]

I believe this will work even if you don't haveGetUser permission.

Saw this in https://aws-blog.de/2021/08/iam-what-happens-when-you-assume-a-role.html

https://docs.aws.amazon.com/STS/latest/APIReference/API_GetCallerIdentity.html says:

No permissions are required to perform this operation. If an administrator adds a policy to your IAM user or role that explicitly denies access to the sts:GetCallerIdentity action, you can still perform this operation. Permissions are not required because the same information is returned when an IAM user or role is denied access.

[simonw]

New output:

% s3-credentials whoami
{
    "UserId": "AIDAWXFXAIOZDWC2E6I3G",
    "Account": "462092780466",
    "Arn": "arn:aws:iam::462092780466:user/simon-administrator"
}