Skip the preview feed if not on a preview .NET (#1957)

scripts/azure-templates-build.yml

@@ -123,7 +123,11 @@ jobs:
             - pwsh: .\scripts\select-vs.ps1
               displayName: Select Visual Studio
           - ${{ if not(endsWith(parameters.name, '_linux')) }}:
-            - pwsh: .\scripts\install-dotnet-workloads.ps1 -InstallDir "$env:AGENT_TOOLSDIRECTORY/dotnet" -SourceUrl "$env:DOTNET_WORKLOAD_SOURCE"
+            - pwsh: .\scripts\install-dotnet-workloads.ps1 -InstallDir "$env:AGENT_TOOLSDIRECTORY/dotnet" -SourceUrl "$env:DOTNET_WORKLOAD_SOURCE" -IsPreview $true
+              condition: ne(variables.DOTNET_VERSION_PREVIEW, '')
+              displayName: Install the preview .NET Core workloads
+            - pwsh: .\scripts\install-dotnet-workloads.ps1 -InstallDir "$env:AGENT_TOOLSDIRECTORY/dotnet" -SourceUrl "$env:DOTNET_WORKLOAD_SOURCE" -IsPreview $false
+              condition: eq(variables.DOTNET_VERSION_PREVIEW, '')
               displayName: Install the .NET Core workloads
           # display dotnet info
           - pwsh: dotnet --info

scripts/install-dotnet-workloads.ps1

@@ -1,17 +1,23 @@
 Param(
     [string] $InstallDir,
-    [string] $SourceUrl
+    [string] $SourceUrl,
+    [boolean] $IsPreview = $true
 )
 
 $ErrorActionPreference = 'Stop'
 
 $env:DOTNET_ROOT="$InstallDir"
 
+$previewFeed = 'https://api.nuget.org/v3/index.json'
+if ($IsPreview) {
+  $previewFeed = 'https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json'
+}
+
 Write-Host "Installing workloads..."
 & dotnet workload install `
   android ios tvos macos maccatalyst wasm-tools maui `
   --from-rollback-file $SourceUrl `
   --source https://api.nuget.org/v3/index.json `
-  --source https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json
+  --source $previewFeed
 
 exit $LASTEXITCODE