Update the audit to prefer ftpmirror.gnu.org over ftp.gnu.org as
suggested by GNU [1]:
> We recommend using this generic ftpmirror.gnu.org address wherever
> possible in links, documentation, etc., to reduce load on the main GNU
> server.
The audit is temporarily disabled to allow migrating all homebrew/core
formulae first.
Should help to address #20456.
[1]: https://www.gnu.org/server/mirror.en.html
049e8cc61c