Rework the way programs are added to the image

Each bundle of programs should be defined using the NixOS module system:

cri.programs = {
  c = [ pkgs.gcc /* ...*/ ];
  java = [ pkgs.java /* ... */ ];
};

and can then be used by each image by doing:

environment.systemPackages = attrValues {
  inherit (config.cri.programs)
    c
    java
  ;
};