Mostly fixed, just a couple more issues

Author: MackinnonBuck

src/ProjectTemplates/TestInfrastructure/PrepareForTest.targets

@@ -26,6 +26,7 @@
     <PropertyGroup>
       <TestTemplateCreationFolder>$([MSBuild]::NormalizePath('$(OutputPath)$(TestTemplateCreationFolder)'))</TestTemplateCreationFolder>
       <CustomTemplateHivePath>$(TestTemplateCreationFolder)\Hives\$([System.Guid]::NewGuid())\.templateengine</CustomTemplateHivePath>
+      <TemplateTestDotNetRoot>$(TestTemplateCreationFolder)\dotnet\</TemplateTestDotNetRoot>
     </PropertyGroup>
 
     <ItemGroup>
@@ -48,10 +49,16 @@
         <_Parameter1>TestTemplateCreationFolder</_Parameter1>
         <_Parameter2>$(TestTemplateCreationFolder)</_Parameter2>
       </AssemblyAttribute>
+
       <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute">
         <_Parameter1>CustomTemplateHivePath</_Parameter1>
         <_Parameter2>$(CustomTemplateHivePath)</_Parameter2>
       </AssemblyAttribute>
+
+      <AssemblyAttribute Include="System.Reflection.AssemblyMetadataAttribute">
+        <_Parameter1>DotNetExeOverride</_Parameter1>
+        <_Parameter2>$(TemplateTestDotNetRoot)dotnet.exe</_Parameter2>
+      </AssemblyAttribute>
     </ItemGroup>
 
     <Message Importance="high" Text="Preparing environment for tests" />
@@ -61,6 +68,7 @@
 
     <ItemGroup>
       <_ExistingFilesFromLastRun Include="$(TestTemplateCreationFolder)**\*" />
+      <!-- <_ExistingFilesFromLastRun Include="$(TemplateTestDotNetRoot)**\*" /> -->
     </ItemGroup>
 
     <Delete Files="@(_ExistingFilesFromLastRun)" ContinueOnError="true" />
@@ -72,6 +80,20 @@
       <Output TaskParameter="RemovedDirectories" ItemName="_CleanedUpDirectories" />
     </RemoveDir>
 
+    <ItemGroup>
+      <_FilesToCopy Include="$(LocalDotNetRoot)dotnet.exe" />
+      <_FilesToCopy Include="$(LocalDotNetRoot)host\**\*" DestinationRelativeFolder="host\" />
+      <_FilesToCopy Include="$(LocalDotNetRoot)shared\**\*" DestinationRelativeFolder="shared\" />
+      <_FilesToCopy Include="$(LocalDotNetRoot)sdk\**\*" DestinationRelativeFolder="sdk\" />
+      <_FilesToCopy Include="$(SharedFrameworkLayoutRoot)\**\*" />
+
+      <_DestinationFiles Include="@(_FilesToCopy->'$(TemplateTestDotNetRoot)\%(DestinationRelativeFolder)%(RecursiveDir)%(Filename)%(Extension)')" />
+    </ItemGroup>
+
+    <Copy SourceFiles="@(_FilesToCopy)"
+          DestinationFiles="@(_DestinationFiles)"
+          SkipUnchangedFiles="true" />
+
     <Message Importance="high" Text="Removed directory %(_CleanedUpDirectories.Identity)" />
 
     <Message Importance="high" Text="Created directory %(_CreatedDirectories.Identity)" />

src/Shared/CommandLineUtils/Utilities/DotNetMuxer.cs

@@ -9,6 +9,7 @@
 using System;
 using System.Diagnostics;
 using System.IO;
+using System.Reflection;
 using System.Runtime.InteropServices;
 
 namespace Microsoft.Extensions.CommandLineUtils;
@@ -22,7 +23,9 @@ internal static class DotNetMuxer
 
     static DotNetMuxer()
     {
-        MuxerPath = TryFindMuxerPath();
+        var dotNetRootOverride = typeof(DotNetMuxer).Assembly.GetCustomAttributes<AssemblyMetadataAttribute>()
+            .SingleOrDefault(a => a.Key == "DotNetExeOverride")?.Value;
+        MuxerPath = dotNetRootOverride ?? TryFindMuxerPath();
     }
 
     /// <summary>