clickhouse: use system LLVM
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.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs