Nix::SSH: Honor NIX_SSHOPTS environment variable.

This fixes a regression introduced by a5c6347ff06ba09530fdf0e01828aaec89f6ceb6.

NixOps uses `NIX_SSHOPTS` to pass OpenSSH options to `nix-copy-closure` and we were using the variable from the local scope in `connectToRemoteNix() `without concatenating it to/with `@sshOpts`.