AWSShellScript doesn't work in China even when regionName is specified.

**Describe the bug**

**To reproduce**

I'm using an AWS Service connection in AzDO to run an AWSShellScript task like this:

```yaml
steps:
- task: AWSShellScript@1
displayName: "Install Secret"
inputs:
awsCredentials: ${{ variables.awsCredentials }}
regionName: ${{ variables.region }}
scriptType: "inline"
inlineScript: |
echo $AWS_REGION
```
and I get

**InvalidClientTokenId: The security token included in the request is invalid.**

**Expected behavior**

I'd expect the task to honor and work with the regionName set to cn-north-1 or another China region, but it only seems to work if I add an AWS_REGION environment variable.

```yaml
env:
AWS_REGION: ${{ variables.region }}
```

**Screenshots**

**Your Environment**

- On-prem or cloud based?: cloud
- Azure DevOps version: cloud
- AWS Toolkit for Azure DevOps version: 1.13.0

Start by locating the AWSShellScript@1 implementation and the code that maps regionName into the task environment. Reproduce the YAML example with cn-north-1 and compare it with the AWS_REGION workaround. Done means the task honors regionName for China regions without requiring an extra environment variable.

Written by the indexing model from the issue text.