Add DotNetBuildOrchestrator flag (#18306)

src/SourceBuild/content/Directory.Build.props

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <_SuppressSdkImports Condition="'$(DotNetBuildFromSource)' == 'true'">true</_SuppressSdkImports>
     <Configuration Condition="$(Configuration) == ''">Release</Configuration>
+    <DotNetBuildOrchestrator>true</DotNetBuildOrchestrator>
   </PropertyGroup>
 
   <PropertyGroup Label="CalculateTargetOS">
@@ -99,6 +100,11 @@
 
     <!-- TargetFrameworkDefaults.props -->
     <NetCurrent>net9.0</NetCurrent>
+
+    <!-- Set up the build phase since the orchestrator switch is passed.
+         From RepoDefaults.props. -->
+    <DotNetBuild>true</DotNetBuild>
+    <DotNetBuildPhase>Orchestrator</DotNetBuildPhase>
   </PropertyGroup>
 
   <!-- Manually import the Versions.props file when the Arcade SDK isn't used. -->

src/SourceBuild/content/repo-projects/Directory.Build.props

@@ -56,6 +56,8 @@
     <BuildArgs>$(BuildArgs) $(FlagParameterPrefix)configuration $(Configuration)</BuildArgs>
     <BuildArgs>$(BuildArgs) -bl</BuildArgs>
     <BuildArgs>$(BuildArgs) /p:DotNetBuildRepo=true</BuildArgs>
+    <!-- Indicate that the build is being run from the orchestrator -->
+    <BuildArgs>$(BuildArgs) /p:DotNetBuildOrchestrator=true</BuildArgs>
     <BuildArgs Condition="'$(CrossBuild)' == 'true'">$(BuildArgs) /p:CrossBuild=$(CrossBuild)</BuildArgs>
   </PropertyGroup>