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.

BUILD.gn

@@ -1644,6 +1644,7 @@ action("node_version_header") {
 action("generate_node_headers") {
   deps = [ ":generate_config_gypi" ]
   script = "script/node/generate_node_headers.py"
+  args = [ rebase_path("$root_gen_dir") ]
   outputs = [ "$root_gen_dir/node_headers.json" ]
 }
 

script/node/generate_node_headers.py

@@ -8,9 +8,7 @@ from pathlib import Path
 
 SRC_DIR = Path(__file__).resolve().parents[3]
 sys.path.append(os.path.join(SRC_DIR, 'third_party/electron_node/tools'))
-sys.path.append(str(Path(__file__).resolve().parents[1]))  # electron/script/
 
-from lib.util import get_out_dir
 import install
 
 class LoadPythonDictionaryError(Exception):
@@ -56,7 +54,7 @@ if __name__ == '__main__':
                 hs = {'files': sorted(files), 'dest_dir': dest_dir}
                 out['headers'].append(hs)
 
-    root_gen_dir = os.path.join(get_out_dir(), 'gen')
+    root_gen_dir = os.path.abspath(sys.argv[1])
     config_gypi_path = os.path.join(root_gen_dir, 'config.gypi')
     node_headers_dir = os.path.join(root_gen_dir, 'node_headers')