Fix authentication for download-missing-packages workflow

[fballiano]

Fixes #219

Problem

The download-missing-packages workflow was failing because neither composer create-project nor the PHP download script could authenticate with repo.magento.com. The workflow was setting COMPOSER_AUTH as an environment variable, but the PHP script only checks for auth.json files.

Solution

Added a step to create ~/.composer/auth.json from the COMPOSER_AUTH secret before running the scripts. This ensures:

• composer create-project can authenticate when creating Magento projects
• The PHP download script can find credentials at ~/.composer/auth.json