fix lighttpd modules for gitweb

Setting `services.lighttpd.gitweb.enable` to `true` doesn't enable the required lighttpd modules to actually make it work. The problem is that `or` and `||` don't mean the same thing: `or` falls back to the second operand if the first is not defined, whereas `||` is the normal logical operator. When `cfg.cgit.enable` is defined, as `false`, the expressions don't have the desired effect.