mangohud: make false values actually disable (#6299)

Currently the following, will produce `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:
<0575c8eb1f/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};