Handle unset AWS_ACCESS_KEY_ID

[willthames]

Description of changes:

Use aws sts get-caller-identity to check whether AWS CLI is installed, rather than the presence of an unnecessary AWS_ACCESS_KEY_ID variable (instance profiles, AWS CLI profiles, and others are reasons it's not necessary)

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[danielkubat]

thanks for this PR. related issue: https://github.com/awslabs/amazon-eks-ami/issues/527

@micahhausler @M00nF1sh could you please consider this?

[willthames]

I've updated this PR to use a better check now

[cartermckinnon]

We need to overhaul the way creds are handled with Packer in general -- we shouldn't be passing keys via env vars like this. I'm going to close this, because this approach isn't getting to the underlying issue.