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/misc/screensavers/alock/default.nix

@@ -38,7 +38,7 @@ stdenv.mkDerivation rec {
       xscreensaver and never will. It's just for locking the current
       X session.
     '';
-    platforms = with platforms; allBut cygwin;
+    platforms = platforms.unix; # Cygwin had problems at one point
     maintainers = with maintainers; [ ftrvxmtrx chris-martin ];
   };
 }

pkgs/misc/screensavers/xscreensaver/default.nix

@@ -50,7 +50,7 @@ stdenv.mkDerivation rec {
     homepage = https://www.jwz.org/xscreensaver/;
     description = "A set of screensavers";
     maintainers = with stdenv.lib.maintainers; [ raskin ];
-    platforms = with stdenv.lib.platforms; allBut cygwin;
+    platforms = stdenv.lib.platforms.unix; # Once had cygwin problems
     inherit version;
     downloadPage = "https://www.jwz.org/xscreensaver/download.html";
     updateWalker = true;