WIP: Add CI to do daily cargo audit checks.

[nathan-at-least]

This is an untested attempt at the "preventative solution" to #816.

It attempts to run cargo audit every 24 hours. I'm calling it WIP because I don't have confidence it will work well as-is:

• How are failing "cron-like" CI workflows represented? Do they mark main with a red flag or block other merges?
• Are these run steps correct?
• Is there a rustsec action we can use to cut down on run time by skipping the cargo-audit install?
• How can we test that this workflow functions correctly prior to merging (if at all)?

[nuttycom]

Marked as draft until this leaves WIP status.