Use the portable rid for --use-current-runtime (backports #21748)

Author: ayakael

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets

@@ -62,7 +62,7 @@ Copyright (c) .NET Foundation. All rights reserved.
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(UseCurrentRuntimeIdentifier)' == 'true'">
-    <RuntimeIdentifier>$(NETCoreSdkRuntimeIdentifier)</RuntimeIdentifier>
+    <RuntimeIdentifier>$(NETCoreSdkPortableRuntimeIdentifier)</RuntimeIdentifier>
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(PlatformTarget)' == ''">