Explicitly require Nix 1.6

People using Nix < 1.6 previously got an unhelpful "infinite
recursion" error.

default.nix

@@ -1 +1,7 @@
-import ./pkgs/top-level/all-packages.nix
+if ! builtins ? nixVersion || builtins.compareVersions "1.6" builtins.nixVersion == 1 then
+
+  abort "This version of Nixpkgs requires Nix >= 1.6, please upgrade!"
+
+else
+
+  import ./pkgs/top-level/all-packages.nix

pkgs/stdenv/generic/default.nix

@@ -12,12 +12,6 @@ let lib = import ../../../lib; in lib.makeOverridable (
 , extraBuildInputs ? []
 }:
 
-if ! builtins ? langVersion then
-
-  abort "This version of Nixpkgs requires Nix >= 1.2, please upgrade!"
-
-else
-
 let
 
   allowUnfree = config.allowUnfree or true && builtins.getEnv "HYDRA_DISALLOW_UNFREE" != "1";