Clean-up branches created from SDK Automation from test-repo-billy PRs

https://dev.azure.com/azure-sdk/internal/_build/results?buildId=3467231&view=logs&j=011e1ec8-6569-5e69-4f06-baf193d1351e&t=5968f867-3c5e-5cb7-4f2b-615be299b519

```
GraphQL: Could not resolve to a PullRequest with the number of 425. (repository.pullRequest)
##[error]PR '425' not found in repo 'Azure/azure-rest-api-specs'. Skipping...
```

Branch clean-up is failing to delete branches like https://github.com/azure-sdk/azure-powershell/tree/sdkAuto/425/Az.alertsmanagement because they didn't come from the public or Azure/azure-rest-api-specs repo. They instead came from https://github.com/test-repo-billy/azure-rest-api-specs-pr/pull/425. There are also plenty that are coming from the public test-repo-billy/azure-rest-api-specs that are not being cleaned up.

Our branch clean-up today uses branch naming patterns in order to figure out which branches it can clean up and currently `sdkauto/` is the pattern we are using for the public specs repo, see https://github.com/Azure/azure-sdk-tools/blob/main/eng/pipelines/branch-cleanup.yml#L126. We should either update the branch clean-up to understand these other repos or use a unique branch prefix for those repos. I prefer if we can use a unique branch prefix if possible.