nix-copy-closure: verify it works with drvs
nix-copy-closure: verify it works with drvs

Creates test coverage for #4210 and 7cf874c17d466d5cffdb0eb6215fcfe8930ed757

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