[tlbuild] zstd as package compressor

[Johannes Hielscher]

> ///////////////
> 
> Or RPi the current install is glacial, so any speedup would be
> welcome.
> 
> ///////////////
> 
> Simon

If downtime is tolerable, I advise to plug the µSD card into a more
beefy machine and install tlmgr with the binary platforms of both the
host (usually x86_64-linux) as well as armhf-linux, and then plug back
into the RPi/SBC of choice.
A lot of the slowness comes from bad IOPS flash though (but this is
something you cannot tackle with optimisations, but only by better
choice of hardware).