clickhouse: use system LLVM

Building LLVM pieces is a huge contributor to build times, and probably
bloats binary size as well. Fortunately, there's a knob for this
specific thing (-DUNBUNDLED=ON seems broken and requires some libraries
which aren't packaged for Nix at the moment).

Hopefully this will make clickhouse able to build on OfBorg.

pkgs/servers/clickhouse/default.nix

@@ -46,6 +46,7 @@ stdenv.mkDerivation rec {
 
   cmakeFlags = [
     "-DENABLE_TESTS=OFF"
+    "-DUSE_INTERNAL_LLVM_LIBRARY=OFF"
   ];
 
   postInstall = ''