create_update_log_file

Create an empty update log file and return its path.

Callers that advertise the log to a user — "Update log: tail -f " — must use this rather than create_update_log_path, which only computes a path. perform_upgrade refuses some upgrades before ever spawning an installer (unknown or unsupported install method, brew missing from PATH, the pre-release gate), and _run_install_subprocess degrades to not persisting output when the log cannot be opened. In all of those cases the path alone would name a file that never comes into existence, leaving the user tailing an ENOENT.