nixos-enter: Ensures presence of full /sys tree.

This partially reverts a change from e88f2896
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.