[llvm] [CI] Downgrade to clang-20 for libc fullbuild (PR #150246)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

I'm not sure this should be downgraded. If anything it should be upgraded to `clang-22` (although it doesn't look like that exists yet according to `llvm.sh`).

The apt repo issue should be resolved soonish.

Really we probably need to use a container for the libc testing for increased stability.

https://github.com/llvm/llvm-project/pull/150246