Fix archlinux-x86_64 template

These commits address the following issues:
* Reflector can fail without rsync installed first
* The URL pointing to the ISO mirror is broken
* Installing base-devel implies downloading and installing packages already there

I've put them together although they're independent but we can work around that if any of them seems bad.

There's still a problem with `grub-mkconfig` but that should be fixed soon (https://bugs.archlinux.org/index.php?do=details&task_id=37904).