[Github] Add Zlib at build time to CI container (llvm#123489)

This patch adds zlib at build time to the CI container. This is needed
to make an lldb test pass that defaults to using the system lld if it is
not explicitly enabled in LLVM_ENABLE_PROJECTS.

Fixes llvm#123479 (by fixing the proximal cause).

.github/workflows/containers/github-action-ci/Dockerfile

@@ -13,7 +13,8 @@ RUN apt-get update && \
     ninja-build \
     python3 \
     git \
-    curl
+    curl \
+    zlib1g-dev
 
 RUN curl -O -L https://github.com/llvm/llvm-project/archive/refs/tags/llvmorg-$LLVM_VERSION.tar.gz && tar -xf llvmorg-$LLVM_VERSION.tar.gz