Fix compilation error with new open function

fortran/src/include/utils.f90

@@ -42,14 +42,15 @@ type(AnswerT) function get_answer(id) result(answer)
     end function
 
     subroutine init_answer_cache()
+        character(len=32), parameter ::file_name = "answers.tsv"
         character(len=64) :: line
         character(len=32) :: val
         character(len=4) :: id_, type_, length
         integer(i18t) :: i, j
         integer :: ios, line_length, unit_number
 
         cached_answers = AnswerT(0, '', errort)
-        unit_number = open_data_file("answers.tsv")
+        unit_number = open_data_file(file_name)
         line_length = 1
         do while (line_length > 0)
             line = ''