add_warning/CI: New strategy to avoid "unexpected-cfgs" warning

Sometimes we want to enforce different lints depending on whether we're running CI. Until now we've been using #[cfg(not(ci_arti_FOO))] for this, but that approach now seems to be somewhat deprecated, and to cause warnings on nightly. (See #1395 (closed).)

Instead, I'm tweaking the add_warning script so that it can conditionally suppress lines depending on a command-line argument that it receives.

Closes #1395 (closed); this is "approach number 5" from that ticket.