Try to avoid running cargo audit on forks (it fails)

.github/workflows/cargo_audit.yml

@@ -6,6 +6,7 @@ on:
 jobs:
   audit:
     runs-on: ubuntu-18.04
+    if: env.GITHUB_HEAD_REF == 0
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1