This fixes warnings like
warning: Ignoring setting 'auto-allocate-uids' because experimental feature 'auto-allocate-uids' is not enabled
warning: Ignoring setting 'impure-env' because experimental feature 'configurable-impure-env' is not enabled
when using the daemon and the user didn't actually set those settings.
Note: this also hides those settings from `nix config show`, but that
seems a good thing.
0acd783190