CI: Add Ubuntu Clang task

This feels more like a band-aid solution - it still requires very manual
intervention to get the next builds, but at least this way we don't rely
on whatever Clang/libc++ happens to ship with FreeBSD for the only
libc++ testing. It seems like sanitizer builds don't use libc++, but I'd
rather a dedicated Linux/Clang/libc++ task at least.

ci/ubuntu-24.04/Dockerfile

@@ -36,6 +36,8 @@ RUN apt-get update && apt-get -y install \
     unzip \
     wget \
     zlib1g-dev \
+    libc++-dev \
+    libc++abi-dev \
   && apt autoclean \
   && rm -rf /var/lib/apt/lists/*