set APP_KEY environment variable automatically after heroku composer install

Question

i have set up a laravel app with continuous deployment ( travis ci ) on heroku . the project's composer.json has a scripts attribute as follows :

    "scripts": {
        "post-install-cmd": [
            "Illuminate\\Foundation\\ComposerScripts::postInstall",
            "@php artisan clear-compiled",
            "@php artisan optimize",
            "@php artisan migrate --force",
            "@php artisan passport:install --force",
            "@php artisan route:cache",
            "@php artisan config:cache"
        ],
        "post-autoload-dump": [
            "Illuminate\\Foundation\\ComposerScripts::postAutoloadDump",
            "@php artisan package:discover --ansi"
        ],
        "post-root-package-install": [
            "@php -r \"file_exists('.env') || copy('.env.example', '.env');\""
        ],
        "post-create-project-cmd": [
            "heroku config:set APP_KEY=$(php artisan key:generate --show)"
        ]
    }

everything works fine , the project gets deployed normally but , the APP_KEY config var is not set up on heroku by post-create-project-cmd . (i have heroku build pack toolbet added to my heroku app)

why isn't it working as an automated script , but it's working if run the command locally ?