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/data/icons/numix-icon-theme-square/default.nix

@@ -29,7 +29,7 @@ stdenv.mkDerivation rec {
     description = "Numix icon theme (square version)";
     homepage = https://numixproject.org;
     license = licenses.gpl3;
-    platforms = with platforms; allBut darwin;
+    platforms = platforms.linux; # Maybe other non-darwin Unix
     maintainers = with maintainers; [ romildo ];
   };
 }