scripts/run_oldconfig.sh: pretend RUSTC doesn't exist
    
    HAS_RUST and RUSTC_VERSION is set (or unset) depending if rustc exists
    on a machine where run_oldconfig.sh is run. We don't want the config to
    oscillate, so disable rust completely for the time being.
    
    Don't use /bin/false, use nonsense like /nothing/nowhere instead. It
    makes scripts/rust-version.sh NOT to scream about missing output.
    
    If we ever want to support rust, we have to:
    * introduce dummy-tools into rust world (there is no CROSS_COMPILE
      before RUST currently)
    * change ignored configs in rpm/check-for-config-changes