ci: PHP via apt in a bookworm container, retry shopware-cli download

setup-php hangs on the runner even with runner=self-hosted; the CLI download
returned a non-gzip body once under concurrent load.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/spec/shopware-guideline-compliance.md

@@ -38,7 +38,7 @@ Jobs (all required checks on `main`):
 
 1. `validate` - install `shopware-cli`, `extension validate .` (hard gate, no `|| true`).
 2. `package` - `extension package` with `--git-commit "$GITHUB_SHA"`, then assert the zip contains built admin JS (`Resources/public/administration/js`) when an admin entry exists.
-3. `phpunit` - `composer create-project shopware/production` (6.6.x) + MariaDB service, copy the plugin into `custom/plugins`, install/activate, run phpunit. PHP 8.3.
+3. `phpunit` - `composer create-project shopware/production` (6.6.x) + MariaDB service, copy the plugin into `custom/plugins`, install/activate, run phpunit. PHP from the Debian bookworm container (8.2), because `shivammathur/setup-php` hangs on this runner.
 
 ### Release workflow (`.gitea/workflows/release.yml`)