Print out the path to the user-environment for each generation.

Example:
$ nix-env --list-generations
  42   2014-08-15 08:30:27               /nix/store/4fkx4zss4p6y6jwww8qblscf5bya9ga6-user-environment

Use case:
When I roll back and forth between generation, I record the uniq hash for identification. This helps with the release process.