Update backward-compatibility.yml

Author: staabm

.github/workflows/backward-compatibility.yml

@@ -31,7 +31,7 @@ jobs:
         run: "composer install --no-dev --no-interaction --no-progress --no-suggest"
 
       - name: "allow composer plugins"
-        run: "composer config --no-plugins allow-plugins.ocramius/package-versions true"
+        run: "composer config --no-plugins --global allow-plugins.ocramius/package-versions true"
 
       - name: "Install BackwardCompatibilityCheck"
         run: "composer global require --dev roave/backward-compatibility-check"