queryMissing(): Use a bigger thread pool
queryMissing(): Use a bigger thread pool

This is a temporary hack until we make queryMissing() async.

Closes #5324.

NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix