[tlbuild] building luahbtex and lualatex-dev

[Karl Berry]

Hello TL builders - per the request from LaTeX, I've just committed
luahbtex binaries for the platforms that are rebuilt when the sources
change, and changed the lualatex-dev format to use them. I wrote the
main list about it.

The platforms I did not include are:
  aarch64-linux
  armhf-linux
  i386-netbsd amd64-netbsd
  i386-cygwin x86_64-cygwin

Johnanes, Simon, Marc, Ken, if you feel like committing to keeping up
with changes to the source tree and making new luahbtex binaries, that'd
be fine, of course, and we can include them. My guess is that it is not
worth your time and that you may as well wait for the regular TL'20
builds to start (next year).  There are presumably few if any active
testers of lualatex-dev on the above platforms.

There's no way to know how frequent source changes will be, but Luigi is
pretty actively developing and fixing things. --thanks, karl.