Avoid UsingTask when the target does not exist (#42238)

Author: jjonescz

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.Sdk.targets

@@ -232,9 +232,11 @@ Copyright (c) .NET Foundation. All rights reserved.
   </PropertyGroup>
 
   <!-- NOTE: Keep in sync with https://github.com/dotnet/msbuild/blob/main/src/Tasks/Microsoft.Common.tasks -->
-  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.CopyRefAssembly" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" />
-  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.Csc" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" />
-  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.Vbc" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" />
+  <!-- `Condition="Exists('$(RoslynTargetsPath)')` is needed because the package might not be yet downloaded during restore in VS
+        and we do not want to fail the restore phase because of that. -->
+  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.CopyRefAssembly" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" Condition="Exists('$(RoslynTargetsPath)')" />
+  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.Csc" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" Condition="Exists('$(RoslynTargetsPath)')" />
+  <UsingTask TaskName="Microsoft.CodeAnalysis.BuildTasks.Vbc" AssemblyFile="$(RoslynTargetsPath)\Microsoft.Build.Tasks.CodeAnalysis.dll" Condition="Exists('$(RoslynTargetsPath)')" />
 
   <ItemGroup Condition="'$(_NeedToDownloadMicrosoftNetSdkCompilersToolsetPackage)' == 'true'">
     <PackageDownload Include="Microsoft.Net.Sdk.Compilers.Toolset" Version="[$(NETCoreSdkVersion)]" />