github-actions: build cargo-audit from source

The `cargo audit` command no longer seems to be provided
as part of the base image. Install it from source instead.

This should get the continuous integration tests running
again.

.github/workflows/audit.yml

@@ -14,5 +14,8 @@ jobs:
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
 
+      - name: Install
+        run: cargo install --locked [email protected]
+
       - name: Audit
         run: cargo audit