Avoid removing the platform config on every build

.github/workflows/continuous-integration.yml

@@ -87,6 +87,7 @@ jobs:
         run: "echo \"COMPOSER_FLAGS=$COMPOSER_FLAGS --ignore-platform-req=php\" >> $GITHUB_ENV"
 
       - name: "Remove platform config to get latest dependencies for current PHP version when build is not locked"
+        if: "contains(matrix.dependencies, 'highest') || contains(matrix.dependencies, 'lowest')"
         run: "composer config platform --unset"
 
       - name: "Allow alpha releases for latest-deps builds to catch problems earlier"