[networkcfg] fix python crash

open(path, "w") returns a TextIOWrapper,
open(path, "wb") returns a BufferedWriter,
.. neither of which is one of the types that os.chmod()
   consumes. Use fileno(), an integer file-descriptor.

FIXES #2294

src/modules/networkcfg/main.py

@@ -155,7 +155,7 @@ network:
 """
             with open(renderer_file, 'w') as f:
                 f.writelines(nm_renderer)
-                os.chmod(f, 0o600)
+                os.chmod(f.fileno(), 0o600)
 
         # Copy existing Netplan configuration
         for cfg in glob.glob(os.path.join(source_netplan, "*.yaml")):