load config.el from outside of nix store

That way a rebuild is not needed to pick up modifications from it

default.nix

@@ -192,7 +192,11 @@ let
       mkdir -p $out
       cp -r ${doomPrivateDir}/* $out
       chmod u+w $out/config.el
-      cat $extraConfigPath >> $out/config.el
+      cat $extraConfigPath > $out/config.extra.el
+      cat > $out/config.el << EOF
+      (load "${builtins.toString doomPrivateDir}/config.el")
+      (load "$out/config.extra.el")
+      EOF
   '';
 
   # Stage 5: catch-all wrapper capable to run doom-emacs even