tame unexpected_cfgs

src/tools/miri/build.rs

@@ -1,8 +1,10 @@
 fn main() {
     // Don't rebuild miri when nothing changed.
     println!("cargo:rerun-if-changed=build.rs");
-    // Re-export the TARGET environment variable so it can
-    // be accessed by miri.
+    // Re-export the TARGET environment variable so it can be accessed by miri. Needed to know the
+    // "host" triple inside Miri.
     let target = std::env::var("TARGET").unwrap();
     println!("cargo:rustc-env=TARGET={target}");
+    // Allow some cfgs.
+    println!("cargo::rustc-check-cfg=cfg(bootstrap)");
 }