flake.nix: Allow unfree packages

Some packages are unfree, since they are lacking a license. This
makes them usable via the package output of the flake. If you use the
overlay, the configuration from the nixpkgs instance using the overlay
will be respected.

flake.nix

@@ -9,6 +9,7 @@
         import nixpkgs {
           inherit system;
           overlays = [ self.overlay ];
+          config.allowUnfree = true;
         }
       );
     in