fix build with bootstrap compiler

src/tools/miri/src/eval.rs

@@ -458,7 +458,14 @@ pub fn eval_entry<'tcx>(
         panic::resume_unwind(panic_payload)
     });
     // `Ok` can never happen.
+    #[cfg(not(bootstrap))]
     let Err(res) = res;
+    #[cfg(bootstrap)]
+    let res = match res {
+        Err(res) => res,
+        // `Ok` can never happen
+        Ok(never) => match never {},
+    };
 
     // Machine cleanup. Only do this if all threads have terminated; threads that are still running
     // might cause Stacked Borrows errors (https://github.com/rust-lang/miri/issues/2396).