Add non-portable RID to list of known runtime packs instead of overwriting in source-build (dotnet#96858)

In dotnet#75597, for source-build we replace all the RIDs with only the `PackageRID` in the `KnownFrameworkReference` for `NetCurrent` when we should have only added an additional RID.

This was causing issues in crossbuilds of vertical builds. The crossgen and ilc projects that run within the build require a runtime for the build machine's RID, but it wasn't found in `KnownFrameworkReference` and the build failed during restore.

Part of reenabling Crossgen and ILC in arm64 crossbuild verticals dotnet/source-build#3698

Author: jtschuster

Directory.Build.targets

@@ -22,11 +22,13 @@
   for Microsoft.NETCore.App.Runtime.<rid> and Microsoft.NETCore.App.Crossgen2.<rid>.
   -->
   <ItemGroup Condition="'$(DotNetBuildFromSource)' == 'true'">
-    <KnownFrameworkReference Update="@(KnownFrameworkReference->WithMetadataValue('Identity', 'Microsoft.NETCore.App')->WithMetadataValue('TargetFramework', '$(NetCoreAppCurrent)'))">
-      <RuntimePackRuntimeIdentifiers>$(PackageRID)</RuntimePackRuntimeIdentifiers>
+    <KnownFrameworkReference Update="Microsoft.NETCore.App">
+      <RuntimePackRuntimeIdentifiers
+        Condition="'%(TargetFramework)' == '$(NetCoreAppCurrent)'">%(RuntimePackRuntimeIdentifiers);$(PackageRID)</RuntimePackRuntimeIdentifiers>
     </KnownFrameworkReference>
-    <KnownCrossgen2Pack Update="@(KnownCrossgen2Pack->WithMetadataValue('Identity', 'Microsoft.NETCore.App.Crossgen2')->WithMetadataValue('TargetFramework', '$(NetCoreAppCurrent)'))">
-      <Crossgen2RuntimeIdentifiers>$(PackageRID)</Crossgen2RuntimeIdentifiers>
+    <KnownCrossgen2Pack Update="Microsoft.NETCore.App.Crossgen2">
+      <Crossgen2RuntimeIdentifiers
+        Condition="'%(TargetFramework)' == '$(NetCoreAppCurrent)'" >%(Crossgen2RuntimeIdentifiers);$(PackageRID)</Crossgen2RuntimeIdentifiers>
     </KnownCrossgen2Pack>
     <!-- Avoid references to Microsoft.AspNetCore.App.Runtime.<rid> -->
     <KnownFrameworkReference Remove="Microsoft.AspNetCore.App" />