Bumped to 15.09 because older channels, when downloaded from
nixos.org, require firefox to be accessed via `pkgs.firefox`
(cherry picked from commit 75ec68f93a)
This is to simplify remote build configuration. These environment
variables predate nix.conf.
The build hook now has a sensible default (namely build-remote).
The current load is kept in the Nix state directory now.