nixos-container: replace 'kill' command with 'terminate'

My earlier commit to have `nixos-container destroy` use `kill` broke
the `container-imperative` test, see[1]. As suggested by @aszlig,
`machinectl terminate` doesn't have that problem, and is the command
that should have been used to begin with rather then `kill`.

1| https://github.com/NixOS/nixpkgs/commit/60c6c7bc9a0d564cf86af4b1711b33db48cf0d07#commitcomment-18478032