Error out if SelfContained is not specified for Native AOT publish

The SDK has logic to specify SelfContained for PublishAot automatically. Except this doesn't apply when we're using msbuild to run the `Publish` target (`_IsPublishing` is not set): https://github.com/dotnet/sdk/blob/75184c7e763197fa2971954aa5baf70ffd188799/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets#L64-L83

This can lead to a bad failure mode because SelfContained is the thing that ensure ILC gets all the references to assemblies it needs. It will appear to work most of the time even without SelfContained (because ILCompiler packages carries a framework with it), but it will fail for things like ASP.NET.

Enable back the logic that just errors out for this. If someone does `msbuild /t:publish` they'll get an error saying they need to also specify SelfContained instead of some weird failure mode. There was some discussion about this aspect in dotnet/sdk#28714.

Author: MichalStrehovsky

src/tools/illink/src/ILLink.Tasks/build/Microsoft.NET.ILLink.targets

@@ -196,9 +196,8 @@ Copyright (c) .NET Foundation. All rights reserved.
     <!-- The defaults currently root non-framework assemblies, which
          is a no-op for portable apps. If we later support more ways
          to customize the behavior we can allow linking portable apps
-         in some cases. If we're not running ILLink because e.g. this
-         is a NativeAOT app, value of SelfContained doesn't matter. -->
-    <NETSdkError Condition="'$(RunILLink)' != 'false' And '$(SelfContained)' != 'true'" ResourceName="ILLinkNotSupportedError" />
+         in some cases. -->
+    <NETSdkError Condition="'$(SelfContained)' != 'true'" ResourceName="ILLinkNotSupportedError" />
 
     <Warning Condition="'$(SuppressILLinkExplicitPackageReferenceWarning)' != 'true' And
                         '%(PackageReference.Identity)' == 'Microsoft.NET.ILLink.Tasks' And '%(PackageReference.IsImplicitlyDefined)' != 'true'"