Pass CrossBuild to inner builds (#17972)

For cross-arch VMR builds, we check DotnetBuildVertical and CrossBuild properties to disable crossgen and ilc. Until we can get those building properly, we should pass CrossBuild from the outer build to the inner builds to make sure the checks work.

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

@@ -165,6 +165,7 @@
     <StandardSourceBuildArgs>$(StandardSourceBuildArgs) /p:PreviouslySourceBuiltNupkgCacheDir="$(PrebuiltSourceBuiltPackagesPath)"</StandardSourceBuildArgs>
     <StandardSourceBuildArgs>$(StandardSourceBuildArgs) /p:SourceBuildUseMonoRuntime=$(SourceBuildUseMonoRuntime)</StandardSourceBuildArgs>
     <StandardSourceBuildArgs>$(StandardSourceBuildArgs) /p:DotNetBuildVertical=$(DotNetBuildVertical)</StandardSourceBuildArgs>
+    <StandardSourceBuildArgs>$(StandardSourceBuildArgs) /p:CrossBuild=$(CrossBuild)</StandardSourceBuildArgs>
 
     <StandardSourceBuildCommand>$(ProjectDirectory)\build$(ShellExtension)</StandardSourceBuildCommand>
   </PropertyGroup>