Pipeline failed at "Set Label" stage for a few package submissions

I noticed my Pull Request (#280151) passed all the validation but failed at the final "Post Validation" > "Set Label" stage hitting an internal error. So there are no labels on the PR.

```
Response: An error was encountered while processing request. Exception: The service is unavailable.
Exception Message: The remote server returned an error: (503) Server Unavailable. (type WebException)
```

I checked the other currently unlabeled open pull requests, and among them seems like there are a few others this happened to: #279175, #277932, #277417

Maybe there should be some kind of thing to automatically retry after a time if that specific exception occurs at that stage?

Validation Pipeline Run [WinGetSvc-Validation-65-280151-20250802-1](https://dev.azure.com/shine-oss/8b78618a-7973-49d8-9174-4360829d979b/_build/results?buildId=156954)

The "Set Label" run log is:
```
==============================================================================
Task : Set Label
Description : Invoke an Azure Function
Version : 1.220.0
Author : Microsoft Corporation
Help URL : https://docs.microsoft.com/azure/devops/pipelines/tasks/utility/azure-function
==============================================================================
POST ***?code=***&
Request body: {
"operationId": "WinGetSvc-Validation-280151-1",
"buildId": "156954",
"projectId": "8b78618a-7973-49d8-9174-4360829d979b",
"planId": "65bb301e-0b06-4823-a188-f87ab291a109",
"jobId": "eca56c61-fe98-578d-1a75-ce6a4db222c4",
"timelineId": "65bb301e-0b06-4823-a188-f87ab291a109",
"taskInstanceId": "791759d0-9601-549e-3319-d5fa3b13f783",
"repository": "microsoft/winget-pkgs",
"pipelineType": "ValidationPipeline",
"pullRequestNumber": "280151",
"buildSourceBranch": "refs/heads/master"
}
Response Code: 0
Response: An error was encountered while processing request. Exception: The service is unavailable.
Exception Message: The remote server returned an error: (503) Server Unavailable. (type WebException)

```

Use the linked Validation Pipeline run and its "Set Label" task log as the starting point; compare the 503 failure with the other unlabeled pull requests (#279175, #277932, and #277417). Done means the specified service-unavailable case is handled and labels are applied successfully after a transient failure.

Written by the indexing model from the issue text.