message: fix libstd rebuilds due to RUSTFLAGS changes
diff: @@ -335,6 +335,7 @@ path = "lib.rs"
  command.current_dir(&dir);
  command.env("XARGO_HOME", &dir);
  command.env("XARGO_RUST_SRC", &rust_src);
  + command.env_remove("RUSTFLAGS"); // Make sure external `RUSTFLAGS` do not influence the build.
  // Use Miri as rustc to build a libstd compatible with us (and use the right flags).
  // However, when we are running in bootstrap, we cannot just overwrite `RUSTC`,
  // because we still need bootstrap to distinguish between host and target crates.
  