Update for Nix Value::type API changes

This updates hydra to be compatible with Nix https://github.com/NixOS/nix/pull/4355.

Along with https://github.com/NixOS/hydra/pull/840 needed for https://github.com/NixOS/nixpkgs/pull/107909

/cc @edolstra

Created by  Matthew Bauer  on December 29, 2020
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