Revert "nixos-container: use systemd-run instead of nsenter"

This reverts commit 7cb100b6.

See also #83432.

This appears to break at least the `container`-backend of `nixops`: when
running `switch-to-configuration` within `nixos-container run`, the
running `systemd`-instance gets reloaded which appears to kill the
`systemd-run` command and causes `nixos-container run` to hang.

The full issue is reported in the original PR[1].

[1] https://github.com/NixOS/nixpkgs/pull/67332#issuecomment-604145869

(cherry picked from commit 7f1ba606ac632c50efd33c435aa59e69d4f58acc)