Add .NET 8 SDK conditionally for ADO build

.build/azure-templates/run-tests-on-os.yml

@@ -107,6 +107,32 @@ steps:
   displayName: 'Use .NET SDK ${{ parameters.dotNetSdkVersion }} (x86)'
   condition: and(succeeded(), or(contains('${{ parameters.framework }}', 'net8.'), contains('${{ parameters.framework }}', 'net9.')), eq('${{ parameters.vsTestPlatform }}', 'x86'))
 
+- task: UseDotNet@2
+  displayName: 'Use .NET SDK 8.0.404'
+  inputs:
+    packageType: 'sdk'
+    version: '8.0.404'
+    performMultiLevelLookup: '${{ variables.PerformMultiLevelLookup }}'
+  condition: and(succeeded(), contains('${{ parameters.framework }}', 'net8.'))
+
+    # Hack: .NET 8+ no longer installs the x86 bits and they must be installed separately. However, it is not
+    # trivial to get it into the path and to get it to pass the minimum SDK version check in runbuild.ps1.
+    # So, we install it afterward and set the environment variable so the above SDK can delegate to it.
+    # This code only works on Windows.
+- pwsh: |
+    $sdkVersion = '8.0.404'
+    $architecture = '${{ parameters.vsTestPlatform }}'
+    $installScriptPath = "${env:AGENT_TEMPDIRECTORY}/dotnet-install.ps1"
+    $installScriptUrl = "https://raw.githubusercontent.com/dotnet/install-scripts/main/src/dotnet-install.ps1"
+    [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
+    Invoke-WebRequest $installScriptUrl -OutFile $installScriptPath -TimeoutSec 60
+    $installPath = "${env:ProgramFiles(x86)}/dotnet"
+    & $installScriptPath -Version $sdkVersion -Architecture $architecture -InstallDir $installPath
+    Write-Host "##vso[task.setvariable variable=DOTNET_ROOT_X86;]$installPath"
+  displayName: 'Use .NET SDK 8.0.404 (x86)'
+  condition: and(succeeded(), contains('${{ parameters.framework }}', 'net8.'), eq('${{ parameters.vsTestPlatform }}', 'x86'))
+
+
 - task: UseDotNet@2
   displayName: 'Use .NET SDK 6.0.421'
   inputs: