[libc++] Install a more recent CMake on libc++ builders

GitOrigin-RevId: 49644cd941c3bf0668b81c61055e9e9a2b58b99e
diff --git a/utils/docker/debian9/buildbot/install-packages.sh b/utils/docker/debian9/buildbot/install-packages.sh
index 82d1463..56e7c00 100755
--- a/utils/docker/debian9/buildbot/install-packages.sh
+++ b/utils/docker/debian9/buildbot/install-packages.sh
@@ -36,5 +36,5 @@
 
 # Install a recent CMake
 yes | apt-get purge cmake
-wget https://github.com/Kitware/CMake/releases/download/v3.15.2/cmake-3.15.2-Linux-x86_64.sh -O /tmp/install-cmake.sh
+wget https://github.com/Kitware/CMake/releases/download/v3.18.2/cmake-3.18.2-Linux-x86_64.sh -O /tmp/install-cmake.sh
 bash /tmp/install-cmake.sh --prefix=/usr --exclude-subdir --skip-license