[libc++][CI] Adds a new CMake version in Docker. (#71582)

This allows testing the upcoming CMake 3.28 release in the CI. CMake
3.28 will have non-experimental support for C++20 modules. So this would
be a better CMake version for the modular builds.

The goal is to remove CMake 3.27 from the CI when the builder work
properly with 3.28.

Author: mordante

libcxx/utils/ci/Dockerfile

@@ -111,11 +111,16 @@ RUN bash /tmp/install-cmake.sh --prefix=/usr --exclude-subdir --skip-license
 RUN rm /tmp/install-cmake.sh
 
 # Install a newer CMake for modules
-# TODO Remove the duplicated installation when all runtimes can be build with CMake 3.27.
+# TODO Remove the duplicated installation when all runtimes can be build with CMake 3.28.
 RUN wget https://github.com/Kitware/CMake/releases/download/v3.27.1/cmake-3.27.1-linux-x86_64.sh -O /tmp/install-cmake.sh
 RUN bash /tmp/install-cmake.sh --prefix=/opt --exclude-subdir --skip-license
 RUN rm /tmp/install-cmake.sh
 
+RUN wget https://github.com/Kitware/CMake/releases/download/v3.28.0-rc4/cmake-3.28.0-rc4-linux-x86_64.sh -O /tmp/install-cmake.sh
+RUN mkdir /opt/cmake-3.28
+RUN bash /tmp/install-cmake.sh --prefix=/opt/cmake-3.28 --exclude-subdir --skip-license
+RUN rm /tmp/install-cmake.sh
+
 # Change the user to a non-root user, since some of the libc++ tests
 # (e.g. filesystem) require running as non-root. Also setup passwordless sudo.
 RUN apt-get update && apt-get install -y sudo