Partially revert "Revert "nixos: remove rsync from base install and add...

Partially revert "Revert "nixos: remove rsync from base install and add explicit path in nixos-install""

This partially reverts commit 0aa75206.

Fine for rsync to be in system path but we still need the explicit path
in nixos-install in case it is invoked from non-NixOS systems and also
to fix OVA test failure

See also https://github.com/NixOS/nixpkgs/commit/0aa75206705afc71b991cceeede644c87088d583

cc @edolstra