Don't save the db if _build doesn't exist

src/build_system.ml

@@ -384,6 +384,7 @@ module Trace = struct
         |> List.map ~f:(fun (path, hash) ->
           Sexp.List [ Atom (Path.to_string path); Atom (Digest.to_hex hash) ]))
     in
+    if Sys.file_exists "_build" then
       write_file file (Sexp.to_string sexp)
 
   let load () =