Unable to copy images to non-default regions with AssumeRole

#### Overview of the Issue

I'm able to successfully assume a role and copy images to multiple regions that are enabled by default in AWS. If I try to copy a region to a non-default region, for example Hong Kong, I will get the following error

```
* Error Copying AMI (ami-0d68130680fad52b4) to region (ap-east-1): AuthFailure: AWS was not able to validate the provided access credentials
```

It seems that Packer is calling the global STS endpoint which issues version 1 tokens which can not be used in non-default regions, see https://repost.aws/knowledge-center/iam-validate-access-credentials

#### Reproduction Steps

Create a configuration with AssumeRole and attempt to copy an image to a non-default region, ie one you must enable explicitly like Hong Kong (ap-east-1)

### Plugin and Packer version

Latest and 1.3.1

Packer should use the regional STS endpoint to ensure version 2 tokens are used