Actually pass through --use-mono-runtime property (#18777)

Author: directhex

src/SourceBuild/content/repo-projects/runtime.proj

@@ -20,6 +20,7 @@
     <BuildArgs Condition="'$(DotNetBuildRuntimeWasmEnableThreads)' == 'true'">$(BuildArgs) /p:DotNetBuildRuntimeWasmEnableThreads=true</BuildArgs>
     <BuildArgs Condition="'$(DotNetBuildRuntimeNativeAOTRuntimePack)' == 'true'">$(BuildArgs) /p:DotNetBuildRuntimeNativeAOTRuntimePack=true</BuildArgs>
     <BuildArgs Condition="'$(PgoInstrument)' == 'true'">$(BuildArgs) $(FlagParameterPrefix)pgoinstrument</BuildArgs>
+    <BuildArgs Condition="'$(SourceBuildUseMonoRuntime)' != ''">$(BuildArgs) /p:SourceBuildUseMonoRuntime=$(SourceBuildUseMonoRuntime)</BuildArgs>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(DotNetBuildSourceOnly)' == 'true'">