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/development/libraries/buddy/default.nix

@@ -18,7 +18,7 @@ stdenv.mkDerivation rec {
     description = "Binary decision diagram package";
     license = "as-is";
 
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
     maintainers = [ stdenv.lib.maintainers.peti ];
   };
 }