fix: Add Rust nightly installation step to security audit workflow

.github/workflows/security-audit.yml

@@ -85,6 +85,8 @@ jobs:
       steps:
         - name: Checkout repository
           uses: actions/checkout@v4
+        - name: Install Rust nightly
+          uses: dtolnay/rust-toolchain@nightly
         - name: Generate code coverage
           run: |
             cargo lockbud -k deadlock