Update Epic prebuilt native toolchain

In line with Change 4339685 in commit 01a13f9d806725796e1476bdab0a7dfeaacf6098, the v12 toolchain with clang 6 is unavailable at the URL expected by the SetupToolchain script (or anywhere else)
https://cdn.unrealengine.com/Toolchain_Linux/native-linux-v12_clang-6.0.1-centos7.tar.gz

I’ve waited several days and am I currently am building successfully with system clang on Arch, but I would appreciate the CentOS toolchain being available soon.

Kind regards!

Thanks for the report; the toolchain will be of course made available before the 4.21 release - I’ll make sure it happens soon.