Fix failing Clang builds on GitHub Actions

[striezel]

A while ago the ubuntu-latest image of GitHub Actions moved from Ubuntu 22.04 to Ubuntu 24.04. See actions/runner-images#10636 for more information on that change.
However, Ubuntu 24.04 does not have packages for Clang 11 and 12 anymore, thus the Clang jobs failed when they tried to install the corresponding compilers. Therefore, the workflow is changed to use ubuntu-22.04 instead of ubuntu-latest.

Note: This could basically be a one line change, just changing runs-on: ubuntu-latest to runs-on: ubuntu-22.04. But if you want to run tests on newer Clang versions, too, then some of them might require Ubuntu 24.04, and in that case this form is easier to extend. For example, one could add the following lines to the job matrix to test more versions:

          - version: 13
            os: ubuntu-22.04
          - version: 14
            os: ubuntu-24.04
          - version: 16
            os: ubuntu-24.04
          - version: 18
            os: ubuntu-24.04

[striezel]

@martinmoene
The MSVC build on Windows 2022 is failing, but it's also failing on the current master (see https://github.com/martinmoene/variant-lite/actions/runs/13053386152/job/36418459317 for example), so that's nothing new and unrelated to the changes in this pull request.

[martinmoene]

@striezel thanks for answering my unquestioned question :)