[tlbuild] updating tlpretest

[Bob Tennent]

I've installed the 2018 pretest. To check whether updates
work, I did

  sudo /usr/local/bin/tlmgr --repository ftp://ftp.math.utah.edu/pub/tlpretest/ update --self --all

and got the message

  TeX Live 2018 is frozen forever and will no
  longer be updated.

which doesn't look right.

Bob T.