Previously, if the currently installed Nix is too old to evaluate
Nixpkgs, then nixos-rebuild would fail and the user had to upgrade Nix
manually. Now, as a fallback, we run ‘nix-store -r’ to obtain a binary
Nix directly from the binary cache.
This allows doing any necessary actions that were not in the installed
nixos-rebuild (such as downloading a new version of Nix). This does
require us to be careful that nixos-rebuild is backwards-compatible
(i.e. can run in any old installation).
Commit 31203732b3104125e107a3141578399cab5d478f dropped the reference to
<nixos> from NIX_PATH (nixos/modules/programs/environment.nix) and thus
prevents systems that are not using channels from rebuilding.
Signed-off-by: aszlig <aszlig@redmoonstudios.org>
