fix(cli): stop leaking a session cache tempdir per build

[gaetschwartz]

Summary

BuildRequest::new builds its session cache dir with TempDir::new().into_path(), which
disarms the guard, so every dx invocation leaves one directory per build target in $TMPDIR
permanently. This restores RAII by holding either an owned Arc<TempDir> or the
caller-supplied --session-cache-dir path.

Regression from #4602, which changed the field from Arc<TempDir> to PathBuf and dropped
flush_session_cache().

// before
let session_cache_dir = args.session_cache_dir.clone()
    .unwrap_or_else(|| TempDir::new().unwrap().into_path());

// after
let session_cache_dir = match args.session_cache_dir.clone() {
    Some(path) => SessionCacheDir::Provided(path),
    None => SessionCacheDir::Owned(Arc::new(TempDir::new().context(..)?)),
};