crystal: fix build (#490981)

      env = {

        LLVM_CONFIG = "${llvmPackages.llvm.dev}/bin/llvm-config";

        FLAGS = [

          "--single-module" # needed for deterministic builds

        ];

        # needed for deterministic builds

        FLAGS = "--single-module";

        # This makes sure we don't keep depending on the previous version of

        # crystal used to build this one.