[BUG]: clang-format installations uninstalls libc++ packages

[miscco]

When trying to install libc++ in our llvm containers, I found that the clang-format installation process uninstalls the previously installed libc++ packages.

I hacked around this by also installing libc++1 with clang-format, but that does not give us the right libc++ version, but the distribution packaged one.

We need a way to install and keep a given version of libc++ that matches the clang version

See e.g https://github.com/rapidsai/devcontainers/actions/runs/19333538446/job/55302370681#step:7:5579

apt-get install -y clang-format-21 llvm-21-tools libc++-21-dev libc++abi-21-dev libunwind-21-dev
Reading package lists...
[2025-11-13T13:46:53.910Z] 

[2025-11-13T13:46:54.063Z] #27 4.895 Building dependency tree...
#27 5.032 Reading state information...
[2025-11-13T13:46:54.214Z] 

[2025-11-13T13:46:54.222Z] #27 5.191 The following additional packages will be installed:

[2025-11-13T13:46:54.337Z] #27 5.192   libc++1 libc++abi1 libclang-cpp21 libllvm21 llvm-libunwind1
#27 5.193 Suggested packages:
#27 5.193   clang
#27 5.215 The following packages will be REMOVED:
#27 5.216   libc++-20-dev libc++abi-20-dev libunwind libunwind-20-dev
#27 5.216 The following NEW packages will be installed:
#27 5.216   clang-format-21 libc++-21-dev libc++abi-21-dev libclang-cpp21 libllvm21
#27 5.217   libunwind-21-dev llvm-21-tools llvm-libunwind1
#27 5.217 The following packages will be upgraded:
#27 5.218   libc++1 libc++abi1
#27 5.307 2 upgraded, 8 newly installed, 4 to remove and 0 not upgraded.
#27 5.307 Need to get 47.9 MB of archives.
#27 5.307 After this operation, 199 MB of additional disk space will be used.
#27 5.307 Get:1 https://apt.llvm.org/noble llvm-toolchain-noble-21/main amd64 libc++abi1 amd64 1:21.1.6~++20251112013458+6ca64545e1eb-1~exp1~20251112133516.60 [120 kB]