nixos-rebuild-ng: change copy closure logic when copying from_host -> to_host

- If user is using Nix 2.18+ (the default), we will switch to use `nix
  copy` since it supports using `--from` and `--to` at the same time
- If user is using older versions of Nix, we will call
  `nix-copy-closure` twice, once with `--from` and another with `--to`

The reason for this change is because the previous logic, that SSH'd to
from_host to copy to to_host using `nix-copy-closure --to` means that
`from_host` needs to have credentials to communicate with `to_host`,
that is not always possible nor expected. It breaks even in simple cases
like `--from` and `--to` is the same host because it is not common for a
host to SSH'd itself. This also makes the behavior more consistent,
since `--from` and `--to` is interpreted from the eval host point of
view EXCEPT when they're used together.

This may of course break in the cases where this old behavior was
assumed, e.g.: deploying with a bastion host. I am not sure how common
this was, and I think this change in behavior should enable more cases
than break, but let's see. For that particular case, running
`nixos-rebuild-ng` inside SSH command (e.g.: `ssh build_host --
nixos-rebuild-ng switch --target-host target_host`) should do the
trick.

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