RBENV_ROOTがミソ

$ sudo RBENV_ROOT=/usr/local/rbenv /usr/local/rbenv/bin/rbenv install x.x.x
$ sudo RBENV_ROOT=/usr/local/rbenv /usr/local/rbenv/bin/rbenv global x.x.x

rbenvで入れたRubyを全ユーザが使えるようにする