Add cargo audit job to security scans

[amika-sq]

#46

cargo audit can find some additional security vulnerabilities than our current FOSSA checks are providing. Currently, we're experiencing known failures in the FOSSA job though, as it's flagging security fixes and licenses, but we don't have resolutions for them yet. We're holding off on adding in this check in the immediate term until we have a path forward to getting those resolved.

Adding additional security checks like this will help harden us from compromised packages.