MINIFICPP-2318 Use libc++ in the clang CI build

[fgerlits]

We used to use libc++ as the standard library in the ubuntu_22_04_clang CI build, but this got accidentally lost in some recent change, and now we build with libstdc++.

This change reverts to using libc++.

https://issues.apache.org/jira/browse/MINIFICPP-2318

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[szaszm]

The ubuntu issue is because "ppa.launchpad.net" is down, so we can't install newer compiler versions. https://status.canonical.com/

[szaszm]

I didn't remember this limitation about cmake vs envvar settings while reviewing last time, but thanks for catching and fixing it.