treewide: get rid of platforms.allBut

Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.

pkgs/games/endless-sky/default.nix

@@ -41,6 +41,6 @@ stdenv.mkDerivation rec {
       gpl3Plus cc-by-sa-30 cc-by-sa-40 publicDomain
     ];
     maintainers = with maintainers; [ lheckemann ];
-    platforms = with platforms; allBut darwin;
+    platforms = platforms.linux; # Maybe other non-darwin Unix
   };
 }