rbinstall.rb: default umask
* tool/rbinstall.rb: revert r49841 and default umask to just
prohibit all from writing. symlink(2) has no argument to set
permissions but is affected by umask.
[ruby-dev:49975] [Bug #13194]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@57903 b2dd03c8-39d4-4d8f-98ff-823fe69b080e