ci: ensure web/sites/default/files is writable during Drupal installation

ci: ensure web/sites/default/files is writable during Drupal installation This change modifies the GitHub Actions workflow for testing Drupal. It ensures that the `web/sites/default/files` directory is created and made writable (using `chmod -R 777`) before the Drupal site installation command is executed. This is crucial for the site installation process, especially when using SQLite as the database, as it needs to write files to this directory. The change addresses two instances within the workflow: 1. In the `Install Drupal site` job for the `apache-trixie` variant, the permissions for `web/sites/default/files` are now set to `777` instead of the previous attempt to set ownership to `www-data:www-data` which might not be sufficient or correctly configured in all container environments. 2. A similar adjustment is made for the primary Drupal installation steps, ensuring the directory is writable. This fix prevents potential installation failures due to file permission issues within the Docker container during the CI build.