Add Fluxer desktop app package

Expose Fluxer as a system package by adding a new fluxer module and
importing it
in the core packages list.

modules/core/packages/fluxer/default.nix

@@ -0,0 +1,6 @@
+{pkgs, ...}: let
+  fluxer =
+    pkgs.callPackage ./fluxer-package.nix {};
+in {
+  environment.systemPackages = [fluxer];
+}