DEFAULT_STDLIB_ROOT is added in initialize already, so it is
duplicated.
before
irb(main):001:0> RBS::Repository.default.dirs
=>
[#<Pathname:/Users/kuwabara.masataka/ghq/github.com/ruby/rbs/stdlib>...</pathname:/users/kuwabara.masataka/ghq/github.com/ruby/rbs/stdlib>