[release] Add zstd archives to github releases (#186526)
This leaves the previous xz files but also creates zstd archives of the
binaries. This provides significantly reduced download sizes. We could
integrate this with cpack but we don't have control over the compression
level in that until version 4.3 which isn't even released yet.
Fixes https://github.com/llvm/llvm-project/issues/164537
