Since nix 2.0 the no-build-hook option was replaced by the builders options
that allows to override remote builders ad-hoc.
Since it is useful to disable remote builders updating nixos without network,
this commit reintroduces the option.
‘nixos-rebuild dry-activate’ builds the new configuration and then
prints what systemd services would be stopped, restarted etc. if the
configuration were actually activated. This could be extended later to
show other activation actions (like uids being deleted).
To prevent confusion, ‘nixos-rebuild dry-run’ has been renamed to
‘nixos-rebuild dry-build’.
