Fix conversion from JSON to fetch attributes
Fix conversion from JSON to fetch attributes

It appears as through the fetch attribute, which
is simply a variant with 3 elements, implicitly
converts boolean arguments to integers. One must
use Explicit<bool> to correctly populate it with
a boolean. This was missing from the implementation,
and resulted in clearly boolean JSON fields being
treated as numbers.

rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix