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/graphics/dcraw/default.nix

@@ -23,7 +23,7 @@ stdenv.mkDerivation rec {
     homepage = http://www.cybercom.net/~dcoffin/dcraw/;
     description = "Decoder for many camera raw picture formats";
     license = stdenv.lib.licenses.free;
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
     maintainers = [ ];
   };
 }