Disable crossgen when poisoning (#42228)

Author: ellahathaway

src/SourceBuild/content/build.proj

@@ -34,6 +34,7 @@
 
     <Message Text="Build Mode: $(BuildModeInfoText)" Importance="high" />
     <Message Text="Build Environment: $(TargetArchitecture) $(Configuration) $(TargetOS) $(TargetRid)" Importance="high" />
+    <Message Condition="'$(EnablePoison)' == 'true'" Text="Crossgen is disabled due to incompatibility with poison checks." Importance="high" />
   </Target>
 
   <Target Name="LogBuildOutputFolders"

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

@@ -40,6 +40,7 @@
     <BuildArgs>$(BuildArgs) /p:UsePortableLinuxSharedFramework=false</BuildArgs>
 
     <BuildArgs Condition="'$(PgoInstrument)' == 'true'">$(BuildArgs) /p:PgoInstrument=true</BuildArgs>
+    <BuildArgs Condition="'$(EnablePoison)' == 'true'">$(BuildArgs) /p:DISABLE_CROSSGEN=true</BuildArgs>
   </PropertyGroup>
 
   <ItemGroup>