[ci] Separating shell and packages in two Nix files.

default.nix

+{ nixpkgs ?
+  builtins.fetchTarball "https://github.com/NixOS/nixpkgs/archive/20.03.tar.gz"
+}:
+let
+  pkgs = import nixpkgs {
+    overlays = [
+      (_: pkgs: {
+
+        hwloc = pkgs.hwloc.overrideAttrs (old: {
+          name = "hwloc-2";
+          src = pkgs.fetchurl {
+            url =
+              "https://download.open-mpi.org/release/hwloc/v2.1/hwloc-2.1.0.tar.gz";
+            sha256 = "0mdqa9w1p6cmli6976v4wi0sw9r4p5prkj7lzfd1877wk11c9c73";
+          };
+        });
+
+        aml = let
+          f = { stdenv, src, autoreconfHook, pkgconfig, numactl, hwloc }:
+            stdenv.mkDerivation {
+              src = ./.;
+              name = "aml";
+              nativeBuildInputs = [ autoreconfHook pkgconfig ];
+              buildInputs = [ hwloc numactl ];
+            };
+        in pkgs.lib.callPackageWith pkgs f { };
+
+      })
+    ];
+  };
+
+in pkgs

shell.nix

 # development shell, includes aml dependencies and dev-related helpers
-# defined by argopkgs nix pkg record
-{ pkgs ? import (
-  builtins.fetchGit {
-    url = "https://xgitlab.cels.anl.gov/argo/argopkgs.git";
-    ref = "master";
-    rev = "56f18ba59462dd7e073417bef9ff15c739905c8c";
-  }) {} }:
+{ pkgs ? import ./. { } }:
 with pkgs;
-pkgs.mkShell {
-	name = "aml";
-	nativeBuildInputs = [ autoreconfHook pkgconfig ];
-        buildInputs = [
-          # dependencies for the code
-          hwloc
-          numactl
-          # deps for docs
-          graphviz
-          doxygen
-          python3Packages.sphinx
-          python3Packages.breathe
-          python3Packages.sphinx_rtd_theme
-          # deps for debug
-          gdb
-          valgrind
-          # style checks
-          (clang-tools.override (o:{llvmPackages=pkgs.llvmPackages_7;}))
-          llvmPackages_7.clang-unwrapped.python
-        ];
-        CFLAGS = "-std=c99 -pedantic -Wall -Wextra -Werror -Wno-unused-but-set-parameter -Wno-builtin-declaration-mismatch";
+mkShell {
+  inputsFrom = [ aml ];
+
+  nativeBuildInputs = [ autoreconfHook pkgconfig ];
+
+  buildInputs = [
+    # deps for docs
+    graphviz
+    doxygen
+    python3Packages.sphinx
+    python3Packages.breathe
+    python3Packages.sphinx_rtd_theme
+    # deps for debug
+    gdb
+    valgrind
+    # style checks
+    (clang-tools.override (o: { llvmPackages = llvmPackages_7; }))
+    llvmPackages_7.clang-unwrapped.python
+  ];
+
+  CFLAGS =
+    "-std=c99 -pedantic -Wall -Wextra -Werror -Wno-unused-but-set-parameter -Wno-builtin-declaration-mismatch";
 }