Use official release builds in update_binaries.sh script

[tcsabina]

As build.quakeworld.nu is finally working and up-to-date, the update_binaries.sh script should get the latest official release binaries from that server, instead of delivering a pre-packed zip file.
It would ease the load on the nquake team, as you guys won't have to update any binary packages anymore. It would just work.

For example, official KTX build: https://builds.quakeworld.nu/ktx/releases/latest/

'latest' always points to the latest release, so no need to update anything afterwards.
This is true for mvdsv and qtv as well