CI: add cargo audit check

[loshz]

Addresses: #187

This PR adds the cargo audit check to a GitHub actions workflow. It runs the check:

• Every time the Cargo.lock or Cargo.toml files are updated in a PR.
• Every Sunday at midnight.

Signed-off-by: Dan Bond danbond@protonmail.com

[mimullin-bbry]

I will appreciate the knowledge that libbpf-rs is periodically monitoring the cargo audit. I have a small nitpick that I think the changes should be squished into a single change.

Also, please see (#148) on how to format the commit message.

[loshz]

Going back to the drawing board on this as I tested it on a personal repo and the action assumes the Cargo.lock and Cargo.toml files are always in the root directory.

There is an open issue that discusses the related changes. I will keep an eye on it and comment on the initial issue #187