### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Currently, when passing `$$.Execution.StartTime` as a parameter to a SFN lambda, it is coming in in the format `HH:MM:SS.###`.

### Expected Behavior

`$$.Execution.StartTime` should be an ISO 8601 datetime string.

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

This is run in a docker compose file.

Step function definition is

```json
{
  "Comment": "Sample SFN",
  "StartAt": "SampleLambda",
  "States": {    
    "SampleLambda": {
      "Type": "Task",
      "Resource": "arn:aws:states:::lambda:invoke",
      "End": true,
      "Parameters": {
        "FunctionName": "Sample-Function",
        "Payload": {
          "data": {
            "startDateTime.$": "$$.Execution.StartTime"
          }
        }
      }
    }
  }
}
```

### Environment

```markdown
- OS: Mac OS Ventura 13.6.1
- LocalStack:
LocalStack version: 3.0.1.dev
LocalStack build date: 2023-11-20
LocalStack build git hash: 3e32b438
```


### Anything else?

Thank you so much for your hard work.
arn_data: ArnData = parse_arn(normalised_state_machine_arn)
        exec_arn = ":".join(
            [
                "arn",
                arn_data["partition"],
                arn_data["service"],
                arn_data["region"],
                arn_data["account"],
                "execution",
                "".join(arn_data["resource"].split(":")[1:]),
                exec_name,
            ]
        )
        if exec_arn in self.get_store(context).executions:
            raise InvalidName()  
        execution = Execution(
            name=exec_name,
            role_arn=state_machine_clone.role_arn,
            exec_arn=exec_arn,
            account_id=context.account_id,
            region_name=context.region,
            state_machine=state_machine_clone,
            start_date=datetime.datetime.now(),
            input_data=input_data,
            trace_header=trace_header,
        )
        self.get_store(context).executions[exec_arn] = execution
        execution.start()
        return execution.to_start_output()
    def describe_execution(