chore: not load plugins for auto-generating docs

[knqyf263]

Description

If a plugin is installed locally, mage docs:genarete also generates references for the plugin subcommand. This PR sets a dummy XDG_DATA_HOME so that the plugin is not loaded.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).