Put miri const eval checking behind -Zmiri
diff --git a/src/bootstrap/check.rs b/src/bootstrap/check.rs
index ee780d1..eee403d 100644
--- a/src/bootstrap/check.rs
+++ b/src/bootstrap/check.rs
@@ -769,6 +769,7 @@
         if build.config.rust_debuginfo_tests {
             flags.push("-g".to_string());
         }
+        flags.push("-Zmiri -Zunstable-options".to_string());
 
         if let Some(linker) = build.linker(target) {
             cmd.arg("--linker").arg(linker);