Merged PR 26805: Build Microsoft.NETCore.Platforms to publish RIDs

Submiting this directly to the internal branch for speed.
I added a comment to indicate that it should never be removed, only commented, since it's a special kind of OOB package.
Fixes dotnet/runtime#76338

src/packages.builds

@@ -26,6 +26,10 @@
     <Project Include="$(MSBuildThisFileDirectory)..\pkg\Microsoft.Private.CoreFx.NETCoreApp\Microsoft.Private.CoreFx.NETCoreApp.builds">
       <AdditionalProperties>$(AdditionalProperties)</AdditionalProperties>
     </Project>
+    <!-- Special package that must be built for new RIDs. Do not remove after branding, only comment it. -->
+    <Project Include="$(MSBuildThisFileDirectory)..\pkg\Microsoft.NETCore.Platforms\Microsoft.NETCore.Platforms.builds">
+      <AdditionalProperties>$(AdditionalProperties)</AdditionalProperties>
+    </Project>
     <!-- add specific builds / pkgproj's here to include in servicing builds -->
   </ItemGroup>