message: use proper platform cache dir
diff: @@ -145,8 +145,9 @@ fn setup(ask_user: bool) {
  }
  }
  - // Next, we need our own libstd. We will do this work in ~/.miri.
  - let dir = directories::UserDirs::new().unwrap().home_dir().join(".miri");
  + // Next, we need our own libstd. We will do this work in whatever is a good cache dir for this platform.
  + let dirs = directories::ProjectDirs::from("miri", "miri", "miri").unwrap();
  + let dir = dirs.cache_dir();
  if !dir.exists() {
  fs::create_dir(&dir).unwrap();
  }
  