message: avoid redundant setting of env vars in phase_runner
diff: @@ -1008,16 +1008,18 @@ fn phase_runner(mut binary_args: impl Iterator<Item = String>, phase: RunnerPhas
  // Set missing env vars. We prefer build-time env vars over run-time ones; see
  // <https://github.com/rust-lang/miri/issues/1661> for the kind of issue that fixes.
  for (name, val) in info.env {
  - if verbose > 0 {
  if let Some(old_val) = env::var_os(&name) {
  - if old_val != val {
  + if old_val == val {
  + // This one did not actually change, no need to re-set it.
  + // (This keeps the `debug_cmd` below more manageable.)
  + continue;
  + } else if verbose > 0 {
  eprintln!(
  "[cargo-miri runner] Overwriting run-time env var {:?}={:?} with build-time value {:?}",
  name, old_val, val
  );
  }
  }
  - }
  cmd.env(name, val);
  }
  