Follow manual to build environment

Use config.nix and documented approach instead of stackoverflow approach

config.nix

@@ -0,0 +1,25 @@
+{
+  # Define packages to install.
+  # Provides convenient list for sharing and porting.
+  # Install via: nix-env -iA nixpkgs.myPackages
+  # See: https://nixos.org/manual/nixpkgs/stable/#sec-building-environment
+  packageOverrides = pkgs: with pkgs; {
+    myPackages = pkgs.buildEnv {
+      name = "my-packages";
+      paths = [
+        # System tools
+        pkgs.dmenu
+        pkgs.st
+        pkgs.dunst
+        pkgs.i3lock-color
+
+        # Dev tools
+        pkgs.litecli
+        pkgs.mkcert
+
+        # Media
+        pkgs.vlc
+      ];
+    };
+  };
+}

configuration.nix

@@ -1,15 +0,0 @@
-let
-  pkgs = import <nixpkgs> {};
-  inherit (pkgs) buildEnv;
-
-in buildEnv {
-  name = "my packages";
-  paths = [  ]
-    # System tools
-    ++ [ pkgs.dmenu pkgs.st pkgs.dunst pkgs.i3lock-color ]
-    # Dev tools
-    ++ [ pkgs.litecli pkgs.mkcert ]
-    # Media
-    ++ [ pkgs.vlc ]
-    ;
-}