Add escape hatch for publishing with SelfContained=true, but no RuntimeIdentifier. (#36047)

Author: nagilson

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

@@ -209,7 +209,7 @@ Copyright (c) .NET Foundation. All rights reserved.
           Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' and '$(HasRuntimeOutput)' == 'true'">
 
     <!-- The following RID errors are asserts, and we don't expect them to ever occur. The error message is added as a safeguard.-->
-    <NETSdkError Condition="'$(SelfContained)' == 'true' and '$(RuntimeIdentifier)' == ''"
+    <NETSdkError Condition="'$(SelfContained)' == 'true' and '$(RuntimeIdentifier)' == '' and '$(AllowSelfContainedWithoutRuntimeIdentifier)' != 'true'"
                  ResourceName="ImplicitRuntimeIdentifierResolutionForPublishPropertyFailed"
                  FormatArguments="SelfContained"/>