Allow build targets to be invoked in specific batches.

Enables building the aspnetcore repo with a single eng\build.cmd invocation (when using desktop msbuild).

Unblocks dotnet/sdk#44828 & dotnet/aspnetcore#58987

src/Microsoft.DotNet.Arcade.Sdk/tools/Build.proj

@@ -44,6 +44,7 @@
 
   <ItemDefinitionGroup>
     <ProjectToBuild>
+      <BuildStep>1</BuildStep>
       <RestoreInParallel>true</RestoreInParallel>
       <BuildInParallel>true</BuildInParallel>
       <DotNetBuildPass>1</DotNetBuildPass>
@@ -277,15 +278,7 @@
              BuildInParallel="%(_ProjectToRestore.RestoreInParallel)"
              Condition="'$(Restore)' == 'true'"/>
 
-    <!--
-      Build solution.
-    -->
-    <MSBuild Projects="@(ProjectToBuild)"
-             Properties="@(_SolutionBuildProps);__BuildPhase=SolutionBuild;_NETCORE_ENGINEERING_TELEMETRY=Build"
-             RemoveProperties="$(_RemoveProps)"
-             Targets="@(_SolutionBuildTargets)"
-             BuildInParallel="%(ProjectToBuild.BuildInParallel)"
-             Condition="'@(_SolutionBuildTargets)' != ''" />
+    <CallTarget Targets="ExecuteBuildTargets" Condition="'@(_SolutionBuildTargets)' != ''" />
 
     <MSBuild Projects="AfterSolutionBuild.proj"
              Properties="@(_CommonProps);_NETCORE_ENGINEERING_TELEMETRY=Build"
@@ -340,4 +333,12 @@
              Condition="'$(Publish)' == 'true' and '$(_ShouldRunPublish)' == 'true'"/>
   </Target>
 
+  <Target Name="ExecuteBuildTargets" Inputs="%(ProjectToBuild.BuildStep)" Outputs="unused">
+    <MSBuild Projects="@(ProjectToBuild)"
+             Properties="@(_SolutionBuildProps);__BuildPhase=SolutionBuild;_NETCORE_ENGINEERING_TELEMETRY=Build"
+             RemoveProperties="$(_RemoveProps)"
+             Targets="@(_SolutionBuildTargets)"
+             BuildInParallel="%(ProjectToBuild.BuildInParallel)" />
+  </Target>
+
 </Project>