Fix cargo-miri not respecting -Zcheck-cfg and lying about it's mode

Author: Urgau

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

@@ -545,6 +545,8 @@ impl Miri {
         cargo.env("MIRI_SYSROOT", &miri_sysroot);
         // Debug things.
         cargo.env("RUST_BACKTRACE", "1");
+        // Manually expect the cfg feature.
+        cargo.rustflag("--check-cfg=cfg(feature,values(any()))");
 
         let mut cargo = Command::from(cargo);
         let _guard = builder.msg(