Don't allow re-assigning Query#validate after validation

[rmosolgo]

Since recently, you could assign Query#validate=... to tell a query whether it should run static validation or not. But the validation result is cached, so it'd be buggy to reassign this afterward. I'd like to warn people about it rather that allow a possible foot-gun. If this doesn't work, this other option is to bust the validation cache whenever this value is reassigned.

Fixes #3901