Update NixOS layer banner

The NixOS logo has changed since the NixOS layer was made. This commit
replaces the layers banner with the current official NixOS logo.

Source of the new logo: https://github.com/NixOS/nixos-artwork/tree/master/logo

CHANGELOG.develop

@@ -2118,6 +2118,7 @@ Other:
 - Disabled Electric-Indent mode, which interfered with the nix-mode indent
   function (thanks to Profpatsch)
 - Associate nix-mode with .nix files (thanks to jpathy)
+- Updated layer banner (thanks to kalium)
 **** Nim
 - Added key binding ~SPC m h h~ to show symbol documentation
   (thanks to Valts Liepiņš)

layers/+os/nixos/README.org

@@ -2,7 +2,7 @@
 
 #+TAGS: layer|os
 
-[[file:img/nixos.jpg]]
+[[file:img/nixos.png]]
 
 * Table of Contents                     :TOC_5_gh:noexport:
 - [[#description][Description]]