Migrate away from using actions-rs

[kate-goldenring]

We are updating to latest node versions in #548; however, actions-rs actions (audit-check and toolchain) don't have node16 compatible versions available and the maintainer is gone. @jbpaux advises to switch to other actions-- for example the toolchain: the dtolnay/rust-toolchain one but none for the audit-check.

[jbpaux]

I just saw https://github.com/rustsec/audit-check is a maintained fork for audit-check so we can switch to that.