Add cargo audit lint step

Scan for cargo audit as CI step, which can be run periodically on Gitlab
CI to notify us about security issues.

.gitlab-ci.yml

 image: "archlinux:latest"
 
 before_script:
-  - pacman -Syu --needed --noconfirm rust gcc
+  - pacman -Syu --needed --noconfirm rust gcc cargo-audit
 
 stages:
   - lint
@@ -17,6 +17,11 @@ clippy:
   script:
     - cargo clippy --all -- -D warnings
 
+audit:
+  stage: lint
+  script:
+    - cargo audit
+
 test:
   stage: test
   script: