[tlbuild] zstd as package compressor

[Karl Berry]

    the closest pretest mirror is in the US, 

Maybe your institution could run a pretest mirror?
Or elsewhere in Australia or Oceania? Will, Ross, would that be easy for
either of you to request?

Looking at https://ctan.org/mirrors#Oceania, U Auckland has a CTAN
mirror. Maybe they'd be up for holding the TL pretest as well. I guess I
can get contact info from CTAN if nothing else works out.

The rsync command would be
rsync -a --delete tug.org::tlpretest /your/local/dir

--best, karl.