Installer PR Checks Need Updated Image

See https://dev.azure.com/dnceng-public/public/_build/results?buildId=1290665&view=logs&jobId=e370af34-c017-5758-dedd-b7a54423e801 (public installer-source-build pipeline):

```
##[error]Failed to request agent. Exception Image 1es-ubuntu-2004-open doesn't exist in pool NetCore-Public-XL
,##[error]The remote provider was unable to process the request.
```

