Build aggregates locally

They're so trivial that we don't want to copy all those dependencies
to remote machines.

pkgs/build-support/release/default.nix

@@ -43,6 +43,7 @@ rec {
     { name, constituents, meta ? { } }:
     pkgs.runCommand name
       { inherit constituents meta;
+        preferLocalBuild = true;
         _hydraAggregate = true;
       }
       ''