nixos-rebuild-ng: kill underlying remote process

`nixos-rebuild-ng` explicitly don't allocate a pseudo-TTY for SSH
because this causes lots of issues depending on the use case (for
example, multiplexing multiple SSH sessions).

Sadly not using a pseudo-TTY also cause other issues, like the fact that
using Ctrl+C (SIGINT) doesn't kill the underlying process because SSH
doesn't support it.

We can't really start using pseudo-TTY unless we want to overcomplicate
the code for parsing results (pseudo-TTY mangles the stdout/stderr
together), so we need to handle killing the underlying remote process
manually.

This is what this commit does, when we receive a `KeyboardInterrupt`
exception while calling `run_wrapper`, we will check if it is a remote
process and send a `pkill --full` with the arguments (this should ensure
that we don't kill other process, but we can't guarantee it). This
assumes the user has `procps` installed, but I think it is a safe
assumption since this seems to be a core package.

Sadly nothing we can do if the user doesn't have `procps` installed, the
good thing is that the worst that can happen is that we will silent
fail and the process will stay in background until it finishes.

Fix #403269.

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