mangohud: make false values actually disable

Currently the following, will produces `some_opt=false` in the
rendered config:

```nix
programs.mangohud.settings = {
  some_opt = false;
};
```

With the intention being to disable the option, this would be
incorrect, as per the following stated at:
<https://github.com/flightlessmango/MangoHud/blob/0575c8eb1f61692a2e5ca152c4f8ed70bef55ee2/data/MangoHud.conf#L3C5-L4C1>

> Use some_parameter=0 to disable a parameter (only works with on/off
> parameters)

As such, I changed the rendering to follow this.
This will be output instead: `some_opt=0`

modules/programs/mangohud.nix

@@ -13,7 +13,7 @@ let
       int = toString option;
       float = int;
       path = int;
-      bool = "false";
+      bool = "0"; # "on/off" opts are disabled with `=0`
       string = option;
       list = concatStringsSep "," (lists.forEach option (x: toString x));
     }.${builtins.typeOf option};

tests/modules/programs/mangohud/basic-configuration.conf

@@ -6,7 +6,7 @@ cpu_stats
 cpu_temp
 cpu_text=CPU
 fps_limit=30,60
-legacy_layout=false
+legacy_layout=0
 media_player_name=spotify
 media_player_order=title,artist,album
 output_folder=/home/user/Documents/mangohud