Factor nixpkgs expression out for easier reuse

morph/make-grid.nix

@@ -5,15 +5,7 @@
 # server in the morph network.
 { name, nodes }:
 let
-  # Pin the deployment package-set to a specific version of nixpkgs.  This is
-  # NixOS 19.03 as of Aug 28 2019.  There's nothing special about it.  It's
-  # just recent at the time of development.  It can be upgraded when there is
-  # value in doing so.  Meanwhile, our platform doesn't shift around beneath
-  # us in surprising ways as time passes.
-  pkgs = import (builtins.fetchTarball {
-    url = "https://github.com/NixOS/nixpkgs/archive/3c83ad6ac13b67101cc3e2e07781963a010c1624.tar.gz";
-    sha256 = "0cdq342wrkvkyccygpp1gvwp7hhqg68hljjwld4vjixm901ayy14";
-  }) {};
+  pkgs = import ./nixpkgs.nix { };
   # Load our JSON configuration for later use.
   cfg = pkgs.lib.trivial.importJSON ./grid.config.json;
 in

morph/nixpkgs.nix

+# Pin the deployment package-set to a specific version of nixpkgs.  This is
+# NixOS 19.03 as of Aug 28 2019.  There's nothing special about it.  It's
+# just recent at the time of development.  It can be upgraded when there is
+# value in doing so.  Meanwhile, our platform doesn't shift around beneath
+# us in surprising ways as time passes.
+(builtins.fetchTarball {
+  url = "https://github.com/NixOS/nixpkgs/archive/3c83ad6ac13b67101cc3e2e07781963a010c1624.tar.gz";
+  sha256 = "0cdq342wrkvkyccygpp1gvwp7hhqg68hljjwld4vjixm901ayy14";
+})