feat: Add Mage-OS Mirror build target

[rhoerr]

Currently it's possible to generate a mirror of Magento tagged releases, and it's possible to create a new Mage-OS release, but it's not possible to create a mirror of Mage-OS tagged releases.

We should add the ability to generate a mirror of Mage-OS releases, so that

• Anyone can independently build, verify, and host Mage-OS packages from source
• Partners can host Mage-OS mirror repositories

If possible, we should make it easy to generate and host mirrors of both Mage-OS and Magento side by side. Many of our existing mirror hosts at https://mage-os.org/distribution/#magento-mirrors have the Magento mirror in a /mirror/ directory.