Don't restart container shells in switch-to-configuration
Don't restart container-startup-done
nixos-rebuild: Exec nixos-rebuild from the new Nixpkgs tree

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).
nixos-rebuild: Fallback for upgrading Nix

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.