Figure out a solution for setting PublishRuntimeIdentifier

[rolfbjarne]

We had to stop setting PublishRuntimeIdentifier:

6aa6c0d

because PublishRuntimeIdentifier ends up being appended to RuntimeIdentifiers, which confuses the build when we're single-rid RuntimeIdentifier build:

https://github.com/dotnet/sdk/blob/b3cc7317b3e938b626e04fcb9ba17524a22362e7/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets#L148-L155

So we need to find a solution here, which probably means adding another opt-out for dotnet/sdk's behavior.