flake: don't reinstantiate nixpkgs

See https://zimbatm.com/notes/1000-instances-of-nixpkgs for
motivation.

flake.nix

@@ -45,7 +45,7 @@
       };
     } // utils.lib.eachDefaultSystem (system:
       let
-        pkgs = import nixpkgs { inherit system; };
+        pkgs = nixpkgs.legacyPackages.${system};
         docs = import ./docs {
           inherit pkgs;
           nmdSrc = nmd;