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.
The local get_out_dir() defaulted to 'Testing' instead of 'Default',
causing e test to fail when using a non-Testing build config. Replace
it with the canonical version from script/lib/util.py.
