nixos/systemd-confinment: use /var/empty as chroot mountpoint
nixos/systemd-confinment: use /var/empty as chroot mountpoint

bind mounting directories into the nix-store breaks nix commands.
In particular it introduces character devices that are not supported
by nix-store as valid files in the nix store. Use `/var/empty` instead
which is designated for these kind of use cases. We won't create any
files beause of the tmpfs mounted.

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