Only pass langVersion if snapshot (#561)

* Only pass langVersion if snapshot

`dart bin/update_idl_bindings.dart` is currently broken
when running locally because we use the web package path
to determine the language version to run the generator.
Because that's 3.10, use of private named parameters throws.
We could pass the generator version here instead, but that
logic is already in compileDartMain, so only pass the lang
version if we're running from snapshot.

CI avoided this because it's presumably running from snapshot.

* Use if branch instead