stop accepting nixpkgs as an argument

set the NIX_PATH to select the version of nixpkgs to use

default.nix

@@ -1,5 +1,4 @@
-{ nixpkgs ? <nixpkgs> }:
-import nixpkgs {
+import <nixpkgs> {
   # docker images run on Linux
   system = "x86_64-linux";
   config = {};

release.nix

@@ -1,4 +1,4 @@
 let
-  pkgs = import ./. {};
+  pkgs = import ./.;
 in
 pkgs.docker-nixpkgs

shell.nix

@@ -1,5 +1,4 @@
-{ nixpkgs ? <nixpkgs> }@args:
-with import ./. args;
+with import ./.;
 mkShell {
   buildInputs = [
     jq