fix MIRI_LOG=info not setting the level for miri itself

Author: RalfJung

src/tools/miri/src/bin/miri.rs

@@ -202,7 +202,7 @@ fn rustc_logger_config() -> rustc_log::LoggerConfig {
             // rustc traced, but you can also do `MIRI_LOG=miri=trace,rustc_const_eval::interpret=debug`.
             if tracing::Level::from_str(&var).is_ok() {
                 cfg.filter = Ok(format!(
-                    "rustc_middle::mir::interpret={var},rustc_const_eval::interpret={var}"
+                    "rustc_middle::mir::interpret={var},rustc_const_eval::interpret={var},miri={var}"
                 ));
             } else {
                 cfg.filter = Ok(var);