source-build: don't use crossgen when building for mono runtime. (#44842)

eng/SourceBuild.props

@@ -55,6 +55,7 @@
 
     <PropertyGroup>
       <InnerBuildArgs>$(InnerBuildArgs) /p:SourceBuildRuntimeIdentifier=$(TargetRuntimeIdentifier)</InnerBuildArgs>
+      <InnerBuildArgs Condition="'$(SourceBuildUseMonoRuntime)' == 'true'">$(InnerBuildArgs) /p:CrossgenOutput=false</InnerBuildArgs>
     </PropertyGroup>
   </Target>