[CI] Install clang and libc++ in build containers

[sarnex]

I'm planning to add a build using libc++ to the nightly, so we need clang and libc++ installed.

Installing clang shouldn't cause a problem with the wrong clang being used for LIT testing as we pass the absolute path to the built clang, ex here.

build-e2e:

Run cmake -GNinja -B./build-e2e -S./llvm/sycl/test-e2e \ 
-DCMAKE_CXX_COMPILER="$GITHUB_WORKSPACE/toolchain/bin/clang++" -DLLVM_LIT="$PWD/llvm/llvm/utils/lit/lit.py" 

run-e2e:

  echo PATH=$PWD/toolchain/bin/:$PATH >> $GITHUB_ENV

We need to specify the clang version to install because of some Ubuntu nonsense.