gameplay-misc/true-nights-and-darkness: Fix flag and texture paths

True nights and darkness wasn't using the existing tlad flag, which is also aliased to the true-lights-and-darkness package. I think this was just an oversight in !483 (merged), so I've switched the flag used.

This won't technically respect any setting used before, so maybe it should have a news post or something, but I'm not sure if it's particularly significant given that it's just meant as a patch on true-lights-and-darkness.

I also fixed the sky texture paths since that was brought up recently on the openmw forum thread. It also looks like it was mentioned three months ago on Matrix, but probably got buried and never addressed.