[CI][R] Update ubuntu-clang CI job to use clang 22 to match CRAN #50298

The test-r-linux-as-cran nightly matrix includes rhub/ubuntu-clang, which maps to CRAN's r-devel-linux-x86_64-debian-clang flavor. However, the container uses clang-15 while CRAN's debian-clang now uses clang 22.1.7.

This causes false positive CI failures (e.g. #50294) that don't reflect real CRAN results.

We already have clang22 in the matrix (added in #47206) for the fedora-clang flavor, so one option is to override the compiler in the ubuntu-clang job to use clang 22, similar to what was done for fedora-clang.

We should also file an upstream issue on rhub/containers to update the ubuntu-clang image to a current clang version.