bacon: add default value for settings

Currently you need to specify the settings if you want to use bacon, but
the empty settings works and is the default upstream:

- <https://dystroy.org/bacon/config/>
- [default `prefs.toml`][1]

[1] https://raw.githubusercontent.com/Canop/bacon/main/defaults/default-prefs.toml

modules/programs/bacon.nix

@@ -17,6 +17,7 @@ in {
 
     settings = mkOption {
       type = settingsFormat.type;
+      default = { };
       example = {
         jobs.default = {
           command = [ "cargo" "build" "--all-features" "--color" "always" ];