[CI Failure] .NET install script download fails with HTTP 503 Service Unavailable from builds.dotnet.microsoft.com

## Build Information

Build: https://github.com/microsoft/aspire/actions/runs/29371663504
Build error leg: Tests / Templates-XUnit_Default_NewUpAndBuildSupportProjectTemplatesTests / Templates-XUnit_Default_NewUpAndBuildSupportProjectTemplatesTests (windows-latest)
Pull request: #18735

## Error Message

```
Failed to download file "https://builds.dotnet.microsoft.com/dotnet/scripts/v1/dotnet-install.ps1". Response status code does not indicate success: 503 (Service Unavailable).
D:\a\aspire\aspire\tests\Shared\InstallSdk.targets(37,5): error MSB3923: Failed to download file
```

## Description

.NET install script download fails with HTTP 503 Service Unavailable from builds.dotnet.microsoft.com

**Type**: infra-failure

## Occurrences

| Date | Build | Job | PR |
|------|-------|-----|----|
| 2026-07-14 | [29371663504](https://github.com/microsoft/aspire/actions/runs/29371663504) | Tests / Templates-XUnit_Default_NewUpAndBuildSupportProjectTemplatesTests / Templates-XUnit_Default_NewUpAndBuildSupportProjectTemplatesTests (windows-latest) | #18735 |

Start with tests/Shared/InstallSdk.targets at line 37 and the linked Windows workflow run, then check whether rerunning Templates-XUnit_Default_NewUpAndBuildSupportProjectTemplatesTests reproduces the HTTP 503. The issue does not define a repository-side change or explicit completion condition; investigate what can be verified locally before proposing a fix.

Written by the indexing model from the issue text.