dhallPackages.dhall-grafana: Small simplification
dhallPackages.dhall-grafana: Small simplification

`dhallPackages.callPackage` can provide the `Prelude` package directly,
and this change uses that to simplify the build for `dhall-grafana`

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs