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/tools/misc/mbuffer/default.nix

@@ -18,6 +18,6 @@ stdenv.mkDerivation rec {
     description  = "A tool for buffering data streams with a large set of unique features";
     license = stdenv.lib.licenses.gpl3;
     maintainers = with stdenv.lib.maintainers; [ tokudan ];
-    platforms = with stdenv.lib.platforms; allBut darwin;
+    platforms = stdenv.lib.platforms.linux; # Maybe other non-darwin Unix
   };
 }