ci/actions: fix typo & use the original action

* Apparently GitHub Actions doesn't work like I expected it would with
  job forks. Revert to the original.
* Fix the `actions/checkout` type.

.github/workflows/audit.yml

@@ -10,8 +10,8 @@ jobs:
   audit:
     runs-on: ubuntu-latest
     steps:
-      - uses: action/checkout@v1
-      - uses: rustsec/audit-check@v1
+      - uses: actions/checkout@v1
+      - uses: actions-rs/audit-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}