nixos/tests/grafana: demonstrate how to use the file provider
nixos/tests/grafana: demonstrate how to use the file provider

Yes, it's still a store path, but it's only for demonstration purposes.
Also we now have fewer warnings in the provision test.

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