Desktop: Fixes #9832: Fix user-installed versions of built-in plugins can't access resources in some cases

[personalizedrefrigerator]

Summary

Fixes an issue where built-in plugins would overwrite the unpack directory for a user-installed version of the plugin. In the case of Simple Backup, this caused user-installed versions of the plugin to be unable to access 7Zip.

Fixes #9832.

Testing

1. Download the JPL file for Simple Backup from joplinapp.org/plugins.
2. Use "install from file" in plugin settings to install the JPL file.
3. Restart Joplin
4. Configure Simple Backup to create 7zip-archive backups
5. Create a backup
   • Verify that the plugin can access 7Zip
6. Uninstall the non-built-in copy of Simple Backup and restart Joplin
7. Create a backup
   • Verify that the built-in version of the plugin can access 7Zip.

This has been tested successfully on Ubuntu 23.10.