ci: declare permissions on security_audit workflow (#718)

rustsec/audit-check creates a GitHub issue when new advisories appear in the
Rust dependency tree, so the workflow needs:
  contents: read   for actions/checkout
  issues: write    for the audit-check action to file its tracking issue

Signed-off-by: Arpit Jain <arpitjain099@gmail.com>

Author: arpitjain099

.github/workflows/security_audit.yaml

@@ -2,6 +2,11 @@ name: Security audit
 on:
   schedule:
     - cron: '0 0 * * *'
+
+permissions:
+  contents: read   # actions/checkout
+  issues: write    # rustsec/audit-check opens a tracking issue when new advisories appear
+
 jobs:
   audit:
     runs-on: ubuntu-24.04