Convert nix package list to own overlay

That should be more modern then the manual entry.
It actually allows to reference the "current" package states, e.g. from
other overlays with applied patches.

Installation is now not different from any other derivation:

    nix-env -i my-packages

Follow Thibaut advice, see: https://mamot.fr/@Thib/107717286778025122
and following posts

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 ]
-    ;
-}

overlays/my-packages/default.nix

@@ -0,0 +1,19 @@
+self: super: {
+  myPackages = super.buildEnv {
+    name = "my-packages";
+    paths = [
+      # System tools
+      self.dmenu
+      self.st
+      self.dunst
+      self.i3lock-color
+
+      # Dev tools
+      self.litecli
+      self.mkcert
+
+      # Media
+      self.vlc
+    ];
+  };
+}