nixos/lighttpd: fix 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.

[Bjørn: modify commit message]