[tlu] TLU critical update still faliing

[Richard Koch]

Running it with bash makes no difference.

Lots of stuff to unpack in Karl's message. I think everyone who has
tried to run this update-tlmgr-latest.sh on a Mac has run into the same
trouble. But perhaps the first experiment I should do here is run it
on an Intel machine with a somewhat older version of MacOS. I'll
do that now and report back in 15 minutes.

Dick Koch