rds-deployment-existing-ad DSC extension failing

[rds-deployment-existing-ad](https://github.com/Azure/azure-quickstart-templates/tree/master/application-workloads/rds/rds-deployment-existing-ad)

### Issue Details
I created an DC with an AD environment in my Azure RG and ran the above template to create an RDS environment in the same RG. It failed on the extensions portion due to a script issue:

Correlation ID: 158ce82f-daed-47fd-942b-ffc104ceba98


```JSON
{
"code": "DeploymentFailed",
"message": "At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/DeployOperations for usage details.",
"details": [
{
"code": "VMExtensionProvisioningError",
"message": "VM has reported a failure when processing extension 'sessionhost'. Error message: \"The DSC Extension failed to execute: Error downloading https://raw.githubusercontent.com/Azure/azure-quickstart-templates/master/application-workloads/rds/rds-deployment-existing-ad/DSC/Configuration.zip after 17 attempts: The remote name could not be resolved: 'raw.githubusercontent.com'.\r\nMore information about the failure can be found in the logs located under 'C:\\WindowsAzure\\Logs\\Plugins\\Microsoft.Powershell.DSC\\2.83.2.0' on the VM.\"\r\n\r\nMore information on troubleshooting is available at https://aka.ms/VMExtensionDSCWindowsTroubleshoot "
},
{
"code": "VMExtensionProvisioningError",
"message": "VM has reported a failure when processing extension 'gateway'. Error message: \"The DSC Extension failed to execute: Error downloading https://raw.githubusercontent.com/Azure/azure-quickstart-templates/master/application-workloads/rds/rds-deployment-existing-ad/DSC/Configuration.zip after 17 attempts: The remote name could not be resolved: 'raw.githubusercontent.com'.\r\nMore information about the failure can be found in the logs located under 'C:\\WindowsAzure\\Logs\\Plugins\\Microsoft.Powershell.DSC\\2.83.2.0' on the VM.\"\r\n\r\nMore information on troubleshooting is available at https://aka.ms/VMExtensionDSCWindowsTroubleshoot "
}
]
}
```

It seems strange because I can grab `https://raw.githubusercontent.com/Azure/azure-quickstart-templates/master/application-workloads/rds/rds-deployment-existing-ad/DSC/Configuration.zip` just fine from the server `gw-vm` after RDPing into it:

```PowerShell
$client = New-Object System.Net.WebClient
$client.DownloadFile("https://raw.githubusercontent.com/Azure/azure-quickstart-templates/master/application-workloads/rds/rds-deployment-existing-ad/DSC/Configuration.zip", "C:\Program Files\Common Files\Configuration.zip")
```

### Repro steps (*if necessary, delete otherwise*)
1. Create a DC in your RG (my method: https://github.com/rjmccallumbigl/Azure-ARM-Templates/blob/main/workingDomainController.json)
2. Deploy template into the same RG with same VNET, subnet, etc: https://portal.azure.com/#create/Microsoft.Template/uri/https%3A%2F%2Fraw.githubusercontent.com%2FAzure%2Fazure-quickstart-templates%2Fmaster%2Fapplication-workloads%2Frds%2Frds-deployment-existing-ad%2Fazuredeploy.json

Start with application-workloads/rds/rds-deployment-existing-ad/azuredeploy.json and the DSC/Configuration.zip reference used by the sessionhost and gateway extensions. Reproduce the deployment with the linked existing-AD setup, then inspect the VM extension logs at C:\WindowsAzure\Logs\Plugins\Microsoft.Powershell.DSC\2.83.2.0. Done means both extensions download their configuration and provision successfully.

Written by the indexing model from the issue text.