Enables cargo audit on pull request

.github/workflows/cargo_audit.yml

@@ -2,6 +2,9 @@ name: Security audit
 on:
   schedule:
     - cron: '0 0 * * *'
+  pull_request:
+    types: [opened, synchronize, reopened]
+
 jobs:
   audit:
     runs-on: ubuntu-18.04