Fix download-via-ssh

http://hydra.nixos.org/build/13286020
build-remote.pl: Provide defaults for $NIX_CURRENT_LOAD and $NIX_REMOTE_SYSTEMS
nix-build: Propagate exit status from nix-store -r
Propagate remote timeouts properly
Reduce verbosity
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
Merge branch 'list-generation-paths' of github.com:goodwillcoding/nix into list-generation-paths
Merge branch 'master' of https://github.com/NixOS/nix into list-generation-paths