Add use-registries config option (and deprecate --no-registries flag)

Some people want to avoid using registries at all on their system; Instead
of having to add --no-registries to every command, this commit allows to
set use-registries = false in the config. --no-registries is still allowed
everywhere it was allowed previously, but is now deprecated.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libexpr/flake/flake.hh

@@ -102,7 +102,7 @@ struct LockFlags
 
     /* Whether to use the registries to lookup indirect flake
        references like 'nixpkgs'. */
-    bool useRegistries = true;
+    std::optional<bool> useRegistries = std::nullopt;
 
     /* Whether to apply flake's nixConfig attribute to the configuration */