Fix Clang builds on GitHub Actions

[striezel]

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

Furthermore, a job for Clang 19 (latest Clang version available on Ubuntu 24.04) is added to run tests with a newer version, too.

(This is basically the same pull request as martinmoene/expected-lite#79 or martinmoene/variant-lite#52, but for scope-lite.)

[striezel]

Hmm ... The newly added Clang 19 build fails, because it looks like Clang 19 does not have the scope header: https://github.com/martinmoene/scope-lite/actions/runs/13971215629/job/39113255491?pr=16

[ 66%] Building CXX object test/CMakeFiles/scope-lite-cpp20.t.dir/scope-main.t.cpp.o
In file included from /home/runner/work/scope-lite/scope-lite/test/scope-main.t.cpp:9:
In file included from /home/runner/work/scope-lite/scope-lite/test/scope-main.t.hpp:12:
/home/runner/work/scope-lite/scope-lite/include/nonstd/scope.hpp:101:10: fatal error: 'scope' file not found
  101 | #include <scope>
      |          ^~~~~~~

As far as I can see it uses <experimental/scope> instead. I'll see whether I can make scope-lite detect that, too.

Edit:
The attempt to make scope-lite work with <experimental/scope> is now available in pull request #17.