treewide: Lots of meta.hydraPlatforms -> meta.platforms

In all of these files, there is no meta.platforms but only
meta.hydraPlatforms, which doesn't seem to have any purpose except being
inconsistent.

pkgs/development/libraries/serf/default.nix

@@ -37,6 +37,6 @@ stdenv.mkDerivation rec {
     description = "HTTP client library based on APR";
     license = stdenv.lib.licenses.asl20;
     maintainers = [stdenv.lib.maintainers.raskin];
-    hydraPlatforms = stdenv.lib.platforms.linux ++ stdenv.lib.platforms.darwin;
+    platforms = stdenv.lib.platforms.linux ++ stdenv.lib.platforms.darwin;
   };
 }