Fix GHA invoking cargo audit

We recently got a major version update of the audit-check GHA.

This version invokes:

```console
cargo audit --json --file ./Cargo.lock
````

Which fails, because we don't have a `Cargo.lock` file.

This commit makes sure the `Cargo.lock` is present prior to
invoking `cargo audit`.

Signed-off-by: Flavio Castelli <fcastelli@suse.com>

.github/workflows/security-audit.yml

@@ -19,6 +19,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938 # v4.2.0
+      - name: Generate lockfile
+        run: cargo generate-lockfile
       - uses: rustsec/audit-check@69366f33c96575abad1ee0dba8212993eecbe998 # v2.0.0
         with:
           token: ${{ secrets.GITHUB_TOKEN }}