Instead running pacman outside the chroot, this runs it inside:
1. Download the needed packages (the pacman cache is bind to the host pacman cache).
2. Make a list of packages to be copied to the pacman cache.
3. Copy the packages with rsync.
This solves:
- Uses the packages in the right branch (before it used the host's branch).
- Don't need to redownload packages anymore, saving tons of time.
- pkg_dir_cache and lng_dir_cache are not needed anymore, no need to clean them too.
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