Use retry for 

Last night build failed caused by a 503 Service Unavailable error:
```
  HTTP 503 returned for "https://raw.githubusercontent.com/squizlabs/PHP_Code  
  Sniffer/2.0.0RC4/composer.json".         
```

We might have a look at https://github.com/marketplace/actions/retry-step if we could use it for `composer install` and `phpcq:rebuild`.