fix: pass root_gen_dir from GN to generate_node_headers.py
PR #50828 replaced a local get_out_dir() (defaulting to 'Testing') with
the shared one from script/lib/util.py (defaulting to 'Default').
Neither default is correct because the actual output directory depends
on the active build config. Pass $root_gen_dir from the GN action so
the script always uses the correct path.
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>
