This partially reverts a change from e88f28965a7d76e83478d3ae6fcddc165b1c94f1
which removed the `mount --rbind /sys`.
While true that the activation scripts will mount `sysfs` at `/sys`,
none of the mountpoints lower in the `/sys` tree are handled by the
activation script, which includes `efivarfs`.
This fixes#38477 since it ensures the presence of `efivarfs` in the
`/sys` tree, which is why the systemd-boot installation failed.
14cab1ac57