[llvm] [libc][workflows] fix fullbuild by installing clang-20/clang++-20 (PR #136074)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM. It might be a good idea to look at moving this over to the CI container at some point which would already contain the appropriate toolchain and is actively maintained.

https://github.com/llvm/llvm-project/pull/136074