GH Actions: don't publish the action for forks

Author: jrfnl

.github/workflows/ci.yml

@@ -6,6 +6,7 @@ on:
 name: ci
 jobs:
   build:
+    if: github.repository == 'phpDocumentor/coding-standard'
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2