Opt into Roslyn Toolset package

Our official build must run through MSBuild.exe, and runs
on VS 17.3 today, which delivers Roslyn 4.3. That's not new
enough to run some of the generators required for .NET 7.0.

Work around the discrepancy while we wait to be able to build
on a hosted 17.4 machine.

Author: rainersigwald

Directory.Build.props

@@ -2,5 +2,6 @@
   <ItemGroup>
     <PackageReference Include="Nerdbank.GitVersioning" PrivateAssets="all" IncludeAssets="build; buildMultitargeting" />
     <PackageReference Include="Microsoft.VisualStudioEng.MicroBuild.Core" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.Net.Compilers.Toolset" PrivateAssets="all" />
   </ItemGroup>
 </Project>

Directory.Packages.props

@@ -5,6 +5,7 @@
     <ItemGroup>
         <PackageVersion Include="Microsoft.Build.Utilities.Core" Version="17.3.1" />
         <PackageVersion Include="Microsoft.Build" Version="17.3.1" />
+        <PackageVersion Include="Microsoft.Net.Compilers.Toolset" Version="4.4.0-4.final" />
         <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="17.4.0-preview-20221003-03" />
         <PackageVersion Include="Microsoft.VisualStudioEng.MicroBuild.Core" Version="1.0.0" />
         <PackageVersion Include="MSTest.TestAdapter" Version="2.3.0-preview-20220810-02" />