Update rustsec/audit-check branch name to main

The latest version of the action doesn't generate Cargo.lock
automatically anymore, so I added an extra step to do this.

.github/workflows/ci.yml

@@ -109,7 +109,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      # rustsec/audit-check used to do this automatically
+      - name: Generate Cargo.lock
+        run: cargo generate-lockfile
       # https://github.com/rustsec/audit-check/issues/2
-      - uses: rustsec/audit-check@master
+      - uses: rustsec/audit-check@main
         with:
           token: ${{ secrets.GITHUB_TOKEN }}