BuildCheck: Warn when reading Configuration early

### Summary

Reading Configuration early can be problematic, and can introduce behavior differences between building single project vs building whole solution.

Building a whole solution will set Configuration as a global property, making it available anytime in evaluation (e.g, in Directory.Build.props). But building a single project without explicitly specifying configuration will only have the value available a little bit later and won't be available in Directory.Build.props.

Logic that will try to change output path based on Configuration in Directory.Build.props will suffer from behavior differences between building single project vs building whole solution, which is very bad.