./x miri: fix sysroot build

Author: RalfJung

src/bootstrap/src/core/build_steps/test.rs

@@ -2755,6 +2755,10 @@ impl Step for Crate {
             // `lib.rs` file, and a `lib.miri.rs` file exists in the same folder, we build that
             // instead. But crucially we only do that for the library, not the test builds.
             cargo.env("MIRI_REPLACE_LIBRS_IF_NOT_TEST", "1");
+            // std needs to be built with `-Zforce-unstable-if-unmarked`. For some reason the builder
+            // does not set this directly, but relies on the rustc wrapper to set it, and we are not using
+            // the wrapper -- hence we have to set it ourselves.
+            cargo.rustflag("-Zforce-unstable-if-unmarked");
             cargo
         } else {
             // Also prepare a sysroot for the target.