Allow the creation of proper NIX flake packages based on the opp_env database.

[rhornig]

It would be great if there would be a mode for opp_env where we could specify that a model (or its dependency) should be generated as a proper nix flake (i.e. it should be properly installed in the nix store and be able to event uploaded to the nix cache).

i.e. if a model name is somehow marked, it should be considered as a nix flake based package:

opp_env install inet-4.5 !omnetpp-6.0.1 

would generate omnetpp-6.0.1 as a nix flake, while inet-4.5 would be installed in the workspace. ! is a marker for the nix package generator.