Provide a custom derivation as "configuration"

This allows to have a list the current installed set of derivations.
Instead of using nix-env like apt.

Use `nix-env -i -f configuration.nix` to install this derivation.
It will then install defined "packages".

This eases to maintain current system and port to NixOS or something
else in the future.

configuration.nix

@@ -0,0 +1,15 @@
+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 ]
+    ;
+}