c3 を allow-plugins に追加

- #5291 に伴い, allow-plugins 必須になったため

.github/workflows/coverage.yml

@@ -186,6 +186,7 @@ jobs:
       - name: Setup xdebug
         run: |
           composer require --dev codeception/c3 "2.*"
+          composer config --no-plugins allow-plugins.codeception/c3 true
           sudo phpenmod -s cli xdebug
           sed -i "7a include __DIR__.'/c3.php';" index.php