boost: Fixes disabling for aarch64-linux for <1.59

Between 2b450377bf and the current
revision, the semantics behind "platforms" changed, and removing the
"aarch64-linux" string doesn't work anymore to filter it out.

Instead, blacklist the platform using the (comparatively) new
badPlatforms.

pkgs/development/libraries/boost/generic.nix

@@ -111,7 +111,8 @@ stdenv.mkDerivation {
     description = "Collection of C++ libraries";
     license = stdenv.lib.licenses.boost;
 
-    platforms = (if versionOlder version "1.59" then remove "aarch64-linux" else id) (platforms.unix ++ platforms.windows);
+    platforms = (platforms.unix ++ platforms.windows);
+    badPlatforms = stdenv.lib.optional (versionOlder version "1.59") "aarch64-linux";
     maintainers = with maintainers; [ peti wkennington ];
   };