Interactive TFM selection doesn't evaluate TargetFrameworks extracted from file-based apps

[jjonescz]

The interactive TFM selection added in #51509 (cc @jonathanpeppers) extracts TargetFrameworks property without evaluating it and hence it won't work for files like the following:

#:property TargetFramework=
#:property MyTfm=net9.0
#:property TargetFrameworks=$(MyTfm);net10.0
Console.WriteLine("Hello Multi-TFM");